They may be there for a long time before the right set of circumstances occur.
Darn right. How about this weird one?
I was having trouble a while back (probably about a year ago now) where Firefox on my HTPC did a terrible job rendering fonts. But only certain fonts. In fact, the visual corruption was
not visible in the "browser" part of Firefox, but throughout the UI chrome of the browser itself (so the "File/Edit/View" menus, and the titlebar and whatnot). Fonts in the "browser" part were perfect. All of the fonts in the menus, settings dialog, and everywhere else in the application looked like they were rendered on an 8-bit Atari.
I couldn't figure it out for the longest time. First I suspected one of my Firefox plugins, so I went through and nuked my Firefox install. Then my Firefox profile, and finally tried running it with just a plain-clean installation. I had DPI Scaling enabled on that system (since it is a HTPC), tried turning it off and the problem vanished. So, I chalked it up to a Firefox-specific bug with High DPI mode (even filed a bug report on Bugzilla). But no other applications exhibited the problem (including other browsers like Chrome). I considered switching over to Chrome, despite the fact that I have a strong resistance to giving Google my entire browsing history (they know enough about me, thanks), and the fact that Chrome doesn't have a nice plugin to scale the UI chrome sizes like Firefox does (which makes Firefox decent for HTPC use). One day I was annoyed enough that I tried enabling high DPI mode on one of my other machines that wasn't having the problem, just to test. I set it up identically to the HTPC, and then opened Firefox.
The fonts were perfect.
Googling, googling, googling...
Turns out, it was on obscure Driver Bug in certain versions of the Catalyst drivers for AMD cards. The machine I tested it on was running a months-old version of the drivers which didn't have the bug yet, so didn't exhibit the problem. But the HTPC was running more current drivers, which did have the problem. The wacky thing was, to experience it, you had to have:
1. High DPI mode enabled (more than 100% sized fonts).
2. A newer version of Catalyst with an AMD GPU.
3. A newish version of Firefox, or certain other obscure applications that used the same kind of font rendering system.
Rolling back to the HTPC's driver to the same one I had on the PC downstairs fixed it instantly. Eventually (months and months later), AMD issued a fix, and now I can use the current drivers again.
Driver issues can be bizarre and unexpected.