Unintentionally Hidden Dot-files
Rob Pike (via Hoà V. DINH):
Long ago, as the design of the Unix file system was being worked out, the entries . and .. appeared, to make navigation easier. […] When one typed ls, however, these files appeared, so either Ken or Dennis added a simple test to the program. It was in assembler then, but the code in question was equivalent to something like this:
if (name[0] == '.') continue;[…]
I’m pretty sure the concept of a hidden file was an unintended consequence. It was certainly a mistake.