Slightly off-topic but something I remember from a few years back involving certificates was when
Starship Troopers got a cinematic release (in 1997 I think). It was classified as a 15 and me and a couple of mates went to see it, but when it was released on DVD soon after it earned an 18 rating for the theatrical print. Bizarre!
You don't really see that too often; it's usually the other way around: Look at
Jaws - a PG now!