Schrodinger's Crime

An act that may or may not be illegal dependant on the presence or absence of Law Enforcement

Joe: "Fuck off, you can't do that, that's illegal!"

(Jim, drinking the entire tank of sea monkeys in the pet shop): "No coppers? If the fuzz don't clock it's not illegal, its Schrodinger's Crime"

Haminit

Contraction of Hang On A Minute, arguably more mistake than slang

“Haminit, that’s not a dog, that’s a really big spider”