Unix’s sticky bit (chmod +t) was introduced in early Unix to tell the kernel to keep a program’s text segment in swap space after execution — making frequently-used binaries load faster on slow hardware. By the time hardware made this irrelevant, the sticky bit had acquired a second, entirely unrelated purpose: when set on a directory like /tmp, it prevents users from deleting each other’s files even when they have write permission to the directory. Same flag, completely different behavior depending on context. This is not the only case where Unix reused a concept for something unrelated, but it is one of the more elegant examples of a hack outliving its original purpose by half a century.
