I've noticed a bug in:

 zsh --version
zsh 5.2 (x86_64-apple-darwin15.0.0)

It drove me up the wall before I realized but it appears if in your $PATH
variable you have a path that contains /./ or /../ the tab-completion for
commands will not search that directory.

For example

*export PATH=/Users/jpolansky/test*
which has a command *cmd_test* in it with +x permissions.

$ cmd<tab>
-- it finds cmd_test

But if you have:
*export PATH=/Users/jpolansky/./test*
*or*
*export PATH=/Users/jpolansky/../test*

*#cmd<tab>*
-- fails to find cmd_test

While I realize it's unusual to have . or .. in a path.. it can happen
through scripting and to me this seems like a bug which should be fixed.
Hopefully you will agree.

John