I think inadvertent gear up landings in WWII aircraft like the Spitfire are a relatively rare occurrence - off hand I can think of three in Europe (P-38, P-51 and a Seafire) in the last 40 or 50 years since I really started taking an interest in such types - obviously there may well have been others I've forgotten or never heard about. I vaguely recall another incident where the gear on a P-40 was retracted in error after landing - fortunately the main gear stayed down and only the tailwheel disappeared!
(A call from observant tower personnel works wonders, pretty sure it saved much embarrasment for a Twin Com pilot I saw get to within seconds of expensive scraping noises in front of a large audience way back the late 1970s!)
Most of the gear incidents either involve a problem beforehand or a collapse on landing - two seat Spits seem particularly prone to undercarriage collapses for some reason.