cd /your/yasnippet/dir
mkdir -p snippets/sh-mode
echo "sh-mode" >> snippets/sh-mode/.yas-parents
emacs -Q -L . -l yasnippet.el -e yas-global-mode

Now do M-x sh-mode or open any file in sh-mode. This will cause an error in the post-command-hook which makes it impossible to still execute most commands. After this many commands will cause errors due to the post-command-hook function failing which makes it very hard to recover.

File mode specification error: (error "Lisp nesting exceeds max-lisp-eval-depth'") yas--all-parents: Lisp nesting exceedsmax-lisp-eval-depth'
Error in post-command-hook (yas-global-mode-check-buffers): (error "Lisp nesting exceeds `max-lisp-eval-depth'")

The issue is easily fixed by either deleting the .yas-parents file or fixing the invalid reference in the .yas-parents file. It would be a lot nicer to detect the error and issue an error message instead, though.