One time flying out through Heathrow I set off the security gate. The UK are anal about this stuff, so I had to stand on the stool whilst getting felt up and violated in every orifice, to make sure I wasn't smuggling WMD's. it's pretty demeaning when it happens in front of hundreds of other travelers.
I did want to mutter under my breath "am I going to get charged for these extras?", but didn't fancy being dragged off to a separate room and copping a full probing...