The readline library for example. If I want to use it, I have to GPL my whole program. I probably might not have modified readline at all. Or any other GPL software which gets somehow linked together with my code. Just watch the controversy whether ZFS kernel modules may be delivered as pre-compiled binaries (sources are fully available). The output of compilers and parsers which are GPLed, also are implicitly covered by the GPL, unless there is a clear exemption, like with GCC.

While GPL software which is used as an entity, like Linux itself or applications like Emacs are fine, any closer contact to the software you are trying to sell is problematic.