diff --git a/git-bisect.sh b/git-bisect.sh
index 97ac600873..79de7017e8 100755
--- a/git-bisect.sh
+++ b/git-bisect.sh
@@ -172,6 +172,25 @@ bisect_write() {
 	test -n "$nolog" || echo "git bisect $state $rev" >>"$GIT_DIR/BISECT_LOG"
 }
 
+is_expected_rev() {
+	test -f "$GIT_DIR/BISECT_EXPECTED_REV" &&
+	test "$1" = $(cat "$GIT_DIR/BISECT_EXPECTED_REV")
+}
+
+mark_expected_rev() {
+	echo "$1" > "$GIT_DIR/BISECT_EXPECTED_REV"
+}
+
+check_expected_revs() {
+	for _rev in "$@"; do
+		if ! is_expected_rev "$_rev"; then
+			rm -f "$GIT_DIR/BISECT_ANCESTORS_OK"
+			rm -f "$GIT_DIR/BISECT_EXPECTED_REV"
+			return
+		fi
+	done
+}
+
 bisect_state() {
 	bisect_autostart
 	state=$1
@@ -181,7 +200,8 @@ bisect_state() {
 	1,bad|1,good|1,skip)
 		rev=$(git rev-parse --verify HEAD) ||
 			die "Bad rev input: HEAD"
-		bisect_write "$state" "$rev" ;;
+		bisect_write "$state" "$rev"
+		check_expected_revs "$rev" ;;
 	2,bad|*,good|*,skip)
 		shift
 		eval=''
@@ -191,7 +211,8 @@ bisect_state() {
 				die "Bad rev input: $rev"
 			eval="$eval bisect_write '$state' '$sha'; "
 		done
-		eval "$eval" ;;
+		eval "$eval"
+		check_expected_revs "$@" ;;
 	*,bad)
 		die "'git bisect bad' can take only one argument." ;;
 	*)
@@ -243,33 +264,18 @@ bisect_auto_next() {
 	bisect_next_check && bisect_next || :
 }
 
-eval_rev_list() {
-	_eval="$1"
-
-	eval $_eval
-	res=$?
-
-	if [ $res -ne 0 ]; then
-		echo >&2 "'git rev-list --bisect-vars' failed:"
-		echo >&2 "maybe you mistake good and bad revs?"
-		exit $res
-	fi
-
-	return $res
-}
-
 filter_skipped() {
 	_eval="$1"
 	_skip="$2"
 
 	if [ -z "$_skip" ]; then
-		eval_rev_list "$_eval"
+		eval "$_eval"
 		return
 	fi
 
 	# Let's parse the output of:
 	# "git rev-list --bisect-vars --bisect-all ..."
-	eval_rev_list "$_eval" | while read hash line
+	eval "$_eval" | while read hash line
 	do
 		case "$VARS,$FOUND,$TRIED,$hash" in
 			# We display some vars.
@@ -332,20 +338,133 @@ exit_if_skipped_commits () {
 	fi
 }
 
+bisect_checkout() {
+	_rev="$1"
+	_msg="$2"
+	echo "Bisecting: $_msg"
+	mark_expected_rev "$_rev"
+	git checkout -q "$_rev" || exit
+	git show-branch "$_rev"
+}
+
+is_among() {
+	_rev="$1"
+	_list="$2"
+	case "$_list" in *$_rev*) return 0 ;; esac
+	return 1
+}
+
+handle_bad_merge_base() {
+	_badmb="$1"
+	_good="$2"
+	if is_expected_rev "$_badmb"; then
+		cat >&2 <<EOF
+The merge base $_badmb is bad.
+This means the bug has been fixed between $_badmb and [$_good].
+EOF
+		exit 3
+	else
+		cat >&2 <<EOF
+Some good revs are not ancestor of the bad rev.
+git bisect cannot work properly in this case.
+Maybe you mistake good and bad revs?
+EOF
+		exit 1
+	fi
+}
+
+handle_skipped_merge_base() {
+	_mb="$1"
+	_bad="$2"
+	_good="$3"
+	cat >&2 <<EOF
+Warning: the merge base between $_bad and [$_good] must be skipped.
+So we cannot be sure the first bad commit is between $_mb and $_bad.
+We continue anyway.
+EOF
+}
+
+#
+# "check_merge_bases" checks that merge bases are not "bad".
+#
+# - If one is "good", that's good, we have nothing to do.
+# - If one is "bad", it means the user assumed something wrong
+# and we must exit.
+# - If one is "skipped", we can't know but we should warn.
+# - If we don't know, we should check it out and ask the user to test.
+#
+# In the last case we will return 1, and otherwise 0.
+#
+check_merge_bases() {
+	_bad="$1"
+	_good="$2"
+	_skip="$3"
+	for _mb in $(git merge-base --all $_bad $_good)
+	do
+		if is_among "$_mb" "$_good"; then
+			continue
+		elif test "$_mb" = "$_bad"; then
+			handle_bad_merge_base "$_bad" "$_good"
+		elif is_among "$_mb" "$_skip"; then
+			handle_skipped_merge_base "$_mb" "$_bad" "$_good"
+		else
+			bisect_checkout "$_mb" "a merge base must be tested"
+			return 1
+		fi
+	done
+	return 0
+}
+
+#
+# "check_good_are_ancestors_of_bad" checks that all "good" revs are
+# ancestor of the "bad" rev.
+#
+# If that's not the case, we need to check the merge bases.
+# If a merge base must be tested by the user we return 1 and
+# otherwise 0.
+#
+check_good_are_ancestors_of_bad() {
+	test -f "$GIT_DIR/BISECT_ANCESTORS_OK" &&
+		return
+
+	_bad="$1"
+	_good=$(echo $2 | sed -e 's/\^//g')
+	_skip="$3"
+
+	# Bisecting with no good rev is ok
+	test -z "$_good" && return
+
+	_side=$(git rev-list $_good ^$_bad)
+	if test -n "$_side"; then
+		# Return if a checkout was done
+		check_merge_bases "$_bad" "$_good" "$_skip" || return
+	fi
+
+	: > "$GIT_DIR/BISECT_ANCESTORS_OK"
+
+	return 0
+}
+
 bisect_next() {
 	case "$#" in 0) ;; *) usage ;; esac
 	bisect_autostart
 	bisect_next_check good
 
+	# Get bad, good and skipped revs
+	bad=$(git rev-parse --verify refs/bisect/bad) &&
+	good=$(git for-each-ref --format='^%(objectname)' \
+		"refs/bisect/good-*" | tr '\012' ' ') &&
 	skip=$(git for-each-ref --format='%(objectname)' \
-		"refs/bisect/skip-*" | tr '\012' ' ') || exit
+		"refs/bisect/skip-*" | tr '\012' ' ') &&
+
+	# Maybe some merge bases must be tested first
+	check_good_are_ancestors_of_bad "$bad" "$good" "$skip"
+	# Return now if a checkout has already been done
+	test "$?" -eq "1" && return
 
+	# Get bisection information
 	BISECT_OPT=''
 	test -n "$skip" && BISECT_OPT='--bisect-all'
-
-	bad=$(git rev-parse --verify refs/bisect/bad) &&
-	good=$(git for-each-ref --format='^%(objectname)' \
-		"refs/bisect/good-*" | tr '\012' ' ') &&
 	eval="git rev-list --bisect-vars $BISECT_OPT $good $bad --" &&
 	eval="$eval $(cat "$GIT_DIR/BISECT_NAMES")" &&
 	eval=$(filter_skipped "$eval" "$skip") &&
@@ -366,9 +485,7 @@ bisect_next() {
 	# commit is also a "skip" commit (see above).
 	exit_if_skipped_commits "$bisect_rev"
 
-	echo "Bisecting: $bisect_nr revisions left to test after this"
-	git checkout -q "$bisect_rev" || exit
-	git show-branch "$bisect_rev"
+	bisect_checkout "$bisect_rev" "$bisect_nr revisions left to test after this"
 }
 
 bisect_visualize() {
@@ -415,6 +532,8 @@ bisect_clean_state() {
 	do
 		git update-ref -d $ref $hash || exit
 	done
+	rm -f "$GIT_DIR/BISECT_EXPECTED_REV" &&
+	rm -f "$GIT_DIR/BISECT_ANCESTORS_OK" &&
 	rm -f "$GIT_DIR/BISECT_LOG" &&
 	rm -f "$GIT_DIR/BISECT_NAMES" &&
 	rm -f "$GIT_DIR/BISECT_RUN" &&
diff --git a/t/t6030-bisect-porcelain.sh b/t/t6030-bisect-porcelain.sh
index 36c9a6965f..85fa39cf0b 100755
--- a/t/t6030-bisect-porcelain.sh
+++ b/t/t6030-bisect-porcelain.sh
@@ -350,6 +350,120 @@ test_expect_success 'bisect does not create a "bisect" branch' '
 	git branch -D bisect
 '
 
+# This creates a "side" branch to test "siblings" cases.
+#
+# H1-H2-H3-H4-H5-H6-H7  <--other
+#            \
+#             S5-S6-S7  <--side
+#
+test_expect_success 'side branch creation' '
+	git bisect reset &&
+	git checkout -b side $HASH4 &&
+	add_line_into_file "5(side): first line on a side branch" hello2 &&
+	SIDE_HASH5=$(git rev-parse --verify HEAD) &&
+	add_line_into_file "6(side): second line on a side branch" hello2 &&
+	SIDE_HASH6=$(git rev-parse --verify HEAD) &&
+	add_line_into_file "7(side): third line on a side branch" hello2 &&
+	SIDE_HASH7=$(git rev-parse --verify HEAD)
+'
+
+test_expect_success 'good merge base when good and bad are siblings' '
+	git bisect start "$HASH7" "$SIDE_HASH7" > my_bisect_log.txt &&
+	grep "merge base must be tested" my_bisect_log.txt &&
+	grep $HASH4 my_bisect_log.txt &&
+	git bisect good > my_bisect_log.txt &&
+	test_must_fail grep "merge base must be tested" my_bisect_log.txt &&
+	grep $HASH6 my_bisect_log.txt &&
+	git bisect reset
+'
+test_expect_success 'skipped merge base when good and bad are siblings' '
+	git bisect start "$SIDE_HASH7" "$HASH7" > my_bisect_log.txt &&
+	grep "merge base must be tested" my_bisect_log.txt &&
+	grep $HASH4 my_bisect_log.txt &&
+	git bisect skip > my_bisect_log.txt 2>&1 &&
+	grep "Warning" my_bisect_log.txt &&
+	grep $SIDE_HASH6 my_bisect_log.txt &&
+	git bisect reset
+'
+
+test_expect_success 'bad merge base when good and bad are siblings' '
+	git bisect start "$HASH7" HEAD > my_bisect_log.txt &&
+	grep "merge base must be tested" my_bisect_log.txt &&
+	grep $HASH4 my_bisect_log.txt &&
+	test_must_fail git bisect bad > my_bisect_log.txt 2>&1 &&
+	grep "merge base $HASH4 is bad" my_bisect_log.txt &&
+	grep "fixed between $HASH4 and \[$SIDE_HASH7\]" my_bisect_log.txt &&
+	git bisect reset
+'
+
+# This creates a few more commits (A and B) to test "siblings" cases
+# when a good and a bad rev have many merge bases.
+#
+# We should have the following:
+#
+# H1-H2-H3-H4-H5-H6-H7
+#            \  \     \
+#             S5-A     \
+#              \        \
+#               S6-S7----B
+#
+# And there A and B have 2 merge bases (S5 and H5) that should be
+# reported by "git merge-base --all A B".
+#
+test_expect_success 'many merge bases creation' '
+	git checkout "$SIDE_HASH5" &&
+	git merge -m "merge HASH5 and SIDE_HASH5" "$HASH5" &&
+	A_HASH=$(git rev-parse --verify HEAD) &&
+	git checkout side &&
+	git merge -m "merge HASH7 and SIDE_HASH7" "$HASH7" &&
+	B_HASH=$(git rev-parse --verify HEAD) &&
+	git merge-base --all "$A_HASH" "$B_HASH" > merge_bases.txt &&
+	test $(wc -l < merge_bases.txt) = "2" &&
+	grep "$HASH5" merge_bases.txt &&
+	grep "$SIDE_HASH5" merge_bases.txt
+'
+
+test_expect_success 'good merge bases when good and bad are siblings' '
+	git bisect start "$B_HASH" "$A_HASH" > my_bisect_log.txt &&
+	grep "merge base must be tested" my_bisect_log.txt &&
+	git bisect good > my_bisect_log2.txt &&
+	grep "merge base must be tested" my_bisect_log2.txt &&
+	{
+		{
+			grep "$SIDE_HASH5" my_bisect_log.txt &&
+			grep "$HASH5" my_bisect_log2.txt
+		} || {
+			grep "$SIDE_HASH5" my_bisect_log2.txt &&
+			grep "$HASH5" my_bisect_log.txt
+		}
+	} &&
+	git bisect reset
+'
+
+check_trace() {
+	grep "$1" "$GIT_TRACE" | grep "\^$2" | grep "$3" >/dev/null
+}
+
+test_expect_success 'optimized merge base checks' '
+	GIT_TRACE="$(pwd)/trace.log" &&
+	export GIT_TRACE &&
+	git bisect start "$HASH7" "$SIDE_HASH7" > my_bisect_log.txt &&
+	grep "merge base must be tested" my_bisect_log.txt &&
+	grep "$HASH4" my_bisect_log.txt &&
+	check_trace "rev-list" "$HASH7" "$SIDE_HASH7" &&
+	git bisect good > my_bisect_log2.txt &&
+	test -f ".git/BISECT_ANCESTORS_OK" &&
+	test "$HASH6" = $(git rev-parse --verify HEAD) &&
+	: > "$GIT_TRACE" &&
+	git bisect bad > my_bisect_log3.txt &&
+	test_must_fail check_trace "rev-list" "$HASH6" "$SIDE_HASH7" &&
+	git bisect good "$A_HASH" > my_bisect_log4.txt &&
+	grep "merge base must be tested" my_bisect_log4.txt &&
+	test_must_fail test -f ".git/BISECT_ANCESTORS_OK" &&
+	check_trace "rev-list" "$HASH6" "$A_HASH" &&
+	unset GIT_TRACE
+'
+
 #
 #
 test_done