git bisect v2.6.27 v2.6.26 problem/bug

[Miquel van Smoorenburg <mikevs@xs4all.net>]

I'm trying to nail down a disk statistics issue that was introduced
in 2.6.27, while 2.6.26 was working OK. So I decided to use git bisect.

However, sometimes I end up with a version before 2.6.26:

$ cat .git/BISECT_LOG
git-bisect start
# good: [bce7f793daec3e65ec5c5705d2457b81fe7b5725] Linux 2.6.26
git-bisect good bce7f793daec3e65ec5c5705d2457b81fe7b5725
# bad: [3fa8749e584b55f1180411ab1b51117190bac1e5] Linux 2.6.27
git-bisect bad 3fa8749e584b55f1180411ab1b51117190bac1e5
# bad: [dd9ca5d9be7eba99d685d733e23d5be7110e9556] USB: usb-serial: fix a sparse warning about different signedness
git-bisect bad dd9ca5d9be7eba99d685d733e23d5be7110e9556
# good: [84c3d4aaec3338201b449034beac41635866bddf] Merge commit 'origin/master'
git-bisect good 84c3d4aaec3338201b449034beac41635866bddf

$ head -4 Makefile 
VERSION = 2
PATCHLEVEL = 6
SUBLEVEL = 26
EXTRAVERSION = -rc8

If at this point I do a 'git bisect good' I end up in a 2.6.26
branch, which is good, but after a few bisects I end up at
a version before v2.6.26 (2.6.26-rc5) again, which should be
impossible right ?

Anyway - at the end I end up with a 'good' version that is
2.6.26-rc<something> which is kind of useless. I know that
version up to 2.6.26 are good ...

What am I doing wrong ?

(git version 1.5.6.5 from debian/lenny)

Mike.