diff --git a/Documentation/git-bisect.txt b/Documentation/git-bisect.txt
index 539f37df26..0b8b0ebba7 100644
--- a/Documentation/git-bisect.txt
+++ b/Documentation/git-bisect.txt
@@ -85,7 +85,7 @@ Oh, and then after you want to reset to the original head, do a
 $ git bisect reset
 ------------------------------------------------
 
-to get back to the master branch, instead of being in one of the
+to get back to the original branch, instead of being in one of the
 bisection branches ("git bisect start" will do that for you too,
 actually: it will reset the bisection state, and before it does that
 it checks that you're not using some old bisection branch).
diff --git a/git-bisect.sh b/git-bisect.sh
index d8d9bfde4c..164e8ed81f 100755
--- a/git-bisect.sh
+++ b/git-bisect.sh
@@ -69,14 +69,19 @@ bisect_start() {
 	head=$(GIT_DIR="$GIT_DIR" git symbolic-ref -q HEAD) ||
 	head=$(GIT_DIR="$GIT_DIR" git rev-parse --verify HEAD) ||
 	die "Bad HEAD - I need a HEAD"
+	#
+	# Check that we either already have BISECT_START, or that the
+	# branches bisect, new-bisect don't exist, to not override them.
+	#
+	test -s "$GIT_DIR/BISECT_START" ||
+		if git show-ref --verify -q refs/heads/bisect ||
+		    git show-ref --verify -q refs/heads/new-bisect; then
+			die 'The branches "bisect" and "new-bisect" must not exist.'
+		fi
 	start_head=''
 	case "$head" in
 	refs/heads/bisect)
-		if [ -s "$GIT_DIR/BISECT_START" ]; then
-		    branch=`cat "$GIT_DIR/BISECT_START"`
-		else
-		    branch=master
-		fi
+		branch=`cat "$GIT_DIR/BISECT_START"`
 		git checkout $branch || exit
 		;;
 	refs/heads/*|$_x40)
@@ -219,18 +224,33 @@ 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 $_eval
+		eval_rev_list "$_eval"
 		return
 	fi
 
 	# Let's parse the output of:
 	# "git rev-list --bisect-vars --bisect-all ..."
-	eval $_eval | while read hash line
+	eval_rev_list "$_eval" | while read hash line
 	do
 		case "$VARS,$FOUND,$TRIED,$hash" in
 			# We display some vars.
@@ -328,8 +348,8 @@ bisect_next() {
 	exit_if_skipped_commits "$bisect_rev"
 
 	echo "Bisecting: $bisect_nr revisions left to test after this"
-	git branch -f new-bisect "$bisect_rev"
-	git checkout -q new-bisect || exit
+	git branch -D new-bisect 2> /dev/null
+	git checkout -q -b new-bisect "$bisect_rev" || exit
 	git branch -M new-bisect bisect
 	git show-branch "$bisect_rev"
 }
diff --git a/t/t6030-bisect-porcelain.sh b/t/t6030-bisect-porcelain.sh
index 5e3e5445c7..933f567983 100755
--- a/t/t6030-bisect-porcelain.sh
+++ b/t/t6030-bisect-porcelain.sh
@@ -284,6 +284,31 @@ test_expect_success 'bisect starting with a detached HEAD' '
 
 '
 
+test_expect_success 'bisect refuses to start if branch bisect exists' '
+	git bisect reset &&
+	git branch bisect &&
+	test_must_fail git bisect start &&
+	git branch -d bisect &&
+	git checkout -b bisect &&
+	test_must_fail git bisect start &&
+	git checkout master &&
+	git branch -d bisect
+'
+
+test_expect_success 'bisect refuses to start if branch new-bisect exists' '
+	git bisect reset &&
+	git branch new-bisect &&
+	test_must_fail git bisect start &&
+	git branch -d new-bisect
+'
+
+test_expect_success 'bisect errors out if bad and good are mistaken' '
+	git bisect reset &&
+	test_must_fail git bisect start $HASH2 $HASH4 2> rev_list_error &&
+	grep "mistake good and bad" rev_list_error &&
+	git bisect reset
+'
+
 #
 #
 test_done