This phrase just occurred twice in half an hour or so on Radio Four; once in Brian Walden's Cooke substitute thing, and once in Any Questions, both times in relation to the Danish cartoons clusterbuggerup.
It's a nice cliche. I've used it myself. But what I failed to understand until a few years ago, and what I suspect may be more widely forgotten, is the full meaning of a shout of fire in a theatre before building regulations.
We're not talking about severe inconvenience or discomfort that's worse because it arises from malice; nor even about Walden's "behaving in a way that deliberately puts other people at personal risk". A man who falsely shouted fire in a Georgian London theatre could expect a stampede that killed half a dozen people. This happened quite a few times. Indeed, it was a technique for blackmailing producers. The restriction proposed may therefore be less severe than the phrase now suggests.
Unfortunately for this argument, the source of my revelation was a display in the basement gents' toilet of Sadlers' Wells theatre in 2003, which I now only dimly remember. Confirmation is beyond my Googling; corrections would be greatly welcomed.