1. Start emacs -q 2. Run M-x shell RET 3. Type "rpm -qip /" and hit TAB. 4. Observe the error message "Symbol's value as variable is void: regexp".
Turns out I don't actually have emacs-auto-complete installed, so I guess this is a bug in the base emacs package. Changing component appropriately.
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Apparently no longer a problem in Fedora 21.