There are lots of those footguns in shellscript. One should always try to avoid any shell and rather use python, tcl, perl or powershell. Any criticism one might have about insecure and broken by design languages apply doubly to shell.

A short list of possible problems (of course depending on the shell in question):

spaces in filenames

newlines in filenames

nonprintables in filenames

empty variables and their expansion ([ x$foo = "xsomething" ])

errors in pipes

environment madness

/bin/bash ?= /bin/sh

Arrays or the lack of it

Space separates lists as arrays

#!bash vs. #!/bin/bash vs. #!/usr/bin/env bash vs. #!/usr/sfw/bin/bash vs. ...

Unwritable and unreadable control structures (if [], case, &&,...)

Information leaks via ps

and many others...

Never use shell except to search for and invoke a sensible language. And anything is more sensible, including C, Perl, brainfuck and Basic.