Solaris 8 briefly had a kernel bug such that usleep() didn't sleep at all. I don't recall the details, just that after patching the system and rebooting things were behaving very bizarrely. I eventually realised the sleep command was returning instantly and from that tracked it to the system call, and then found a patch for the issue. Google is failing me in digging up any details. It would've been in the 2000-2004 time frame.