I have had some colleagues who did not get the semantics either, for many features of Unix permission bits, not just the two mentioned above.

For example, some did not know how to use the symbolic form of the chmod command's permission flags, e.g., like ug+rx, only the octal ones, like 660, and would typically indiscriminately use 777, not knowing about the security risks :)

...especially when they don't read documentation as often as they should ;)

ahem

Yes, I mean the setuid bit. The bit that makes the groups work for read/write to a directory. The same bit that's dangerous if it's set for a root user or group, and can be iffy if it's executable... yes that bit.