Here’s something fun I discovered by accident. When you typo a path with double forward slashes in a bash prompt with a default PS1 listing the working directory with \w, you get double slashes:

user@machine /path/$ cd //
user@machine //$

This also happens in sh on Linux and Mac OS X, presumably because sh aliased to bash.

$ cd //
$ pwd
==> //

This is not the case in tcsh:

[machine:/path/] user% cd //
[machine:/] user%

Or with Almquist sh on FreeBSD:

$ cd //
$ pwd
==> /

But it gets funky if we chain shells. If we start with bash, then launch tcsh, the current double slash working directory is preserved:

user@machine /path/$ cd //
user@machine //$ tcsh
[machine://] user%

I haven’t tried with zsh or fish.