The title pretty much says it all. Squid-2.6.STABLE15 has a number of important fixes to the previous version but has a couple of teething problems and may be unstable in the real world.
Leave Squid-2.6.STABLE15 right well alone and wait until Henrik rolls the Squid-2.6.STABLE16 release.
(You can blame me for one of the bugs introduced into Squid-2.6.STABLE14 if you run select(); oops. Henrik’s corrected that in STABLE15 and STABLE16.)
If you run NTLM authentication then please upgrade to STABLE16 and report any bugs or bad behaviour you see. Some people have reported broken behaviour between earlier squid-2.6 STABLE versions and the current STABLE release – if you just stay on the old version and don’t tell us why you’re doing it we can’t fix the bugs!