The scars of my NPTL experience
It's only recently that I've realized how jumpy about certain things my NPTL (the 'native Posix thread library' for Linux) experience still makes me.
NPTL was the new, improved Linux threading library, supplanting the old and less efficient 'pthreads' library. To get its good performance it needed some kernel support, kernel support that was only added in the Linux 2.5 development kernels (and then in Linux 2.6 when 2.5 turned into 2.6).
However, Red Hat was a big NPTL booster and they wanted to use NPTL well before 2.6 was ready and usable. So, starting with Red Hat 8, they hacked NPTL support into the version of the 2.4 kernel that they shipped. Since the system basically required NPTL, this meant that you couldn't really use anything except Red Hat's kernels.
At the time, we wanted (and sometimes needed) to use stock kernels on our servers. So, no Red Hat 8 for us.
This might have been OK if Red Hat had only done it for one release. However, the 2.6 kernel wasn't ready for use until Fedora Core 2; RH 8, Red Hat 9, and Fedora Core 1 were all NPTL-on-2.4 releases. The upshot was that pretty much all of our servers had to stay all the way back at Red Hat 7.3 for quite a while, which really was no fun.
Ever since then I have been perhaps irrationally twitchy about being forced to depend on a specific vendor's kernel hacks, however useful they may be and however little I may really need to run my own kernels on servers any more.