It was always the simpler things that got me. I can remember in one of the movies I think, they were trying to rig up a massove catapult to launch one of them sat in a shopping trolley out a garage door. Once they got it all sorted and ready to fire, one of them quickly pulled the garage door shut and launched the trolley into it. The instant look of fear on the lads face followed by getting fired into the door was priceless. Still tickles me thinking about it.
This and Johnny Knoxville asking if Butterbean was okay are the two ones that always immediately spring to mind.