Got a great story about this guys.
A year or so ago I was at an engagement party of a guy I used to work with. ANother guy from the same workplace was there telling me about his new BMW Z3, and that it was a 3 litre (which I pretty much knew to be untrue)
Anyway, his house was on the way to my girlfriends so I made a note of his registration and looked it up on the dvla website, took a screenshot of it, and posted it to his facebook profile with a note saying "remember you told me your car was a 3 litre? Well take a look at this, should save you some money on insurance now you know it's a 1.9!
Gutted he was.