Saw Infernal Affairs at SIFF last night. I thought of some new entries for the fabulously humorous Ebert's Bigger Little Movie Glossary. Maybe they're already in there, but if not...
Always photocopy your passport and driver's license. In any movie where a policeman goes undercover and only one other person in his department knows of his secret identity (usually his department chief), that one person inevitably gets killed, leaving the mole stranded. Inevitably he has to flee while getting shot at by other policeman, all the while trying to think of some way to prove his identity. (e.g. Face/Off)
Keep your friends close, your enemies closer. In any movie where a gang boss suspects one of his gang of being an undercover cop, the mole is inevitably his right hand man, the one he trusts the most.
Keep your friends close, your enemies closer part II. In any movie where a gang boss suspects one of his gang of being an undercover cop, he merely needs to think back to the member of his group who earned his position by appearing out of nowhere and (a) committing some extravagant act of violence which saves the gang boss's life and simultaneous proves his ruthlessness or (b) executing some other member of the gang who has somehow failed the gang boss or is secretly working for some other gang or (c) offering to step in to kill some policeman or innocent citizen when the gang boss was about to do the job himself (usually the death can't be confirmed because the body is tossed into a river or something like that).