I've updated .so files on FreeBSD while they're running. They weren't busy and a program which had it mmaped to run promptly crashed (my update wasn't intended to be hot loaded and wasn't crafted to be safe, although, it could have been if I knew it was possible). And now I won't forget why I should use install instead of cp (install unlinks before writing, by default, cp opens and overwrites the existing file)

In my experience on Linux, shared libraries can be modified while running (often causing a crash), while executables cannot (ETXTBUSY).