It happens ALL the time at my GW. Everyone does it, it's not that they cheat but because they don't know better (at least most of the time). Even the most experienced player I know there always moves extra. I only know two people whom I haven't seeen overmeasure, and one of them is myself (yeah yeah I'm boasting
It's so common that I don't call people on it. First of all, it's very hard to prove it when he's moved his first model. Then it's just speculating where it was originally.
Second, you quickly get a bad reputation for calling people on it constantly. I prefer being known as easy-going rather than a rules-lawyer, even if I know I'm right and I am disadvantaged by it.
The last time I didn't call on it (or rather, I did, but I gave up eventually because the guy wasn't exactly agreeing with me... and he was probably 20cm taller than me :/ ), I lost the game. His Kroot were able to charge my battlesuit (TL flamer & AFP, if it interests you), even though I knew they shouldn't have been able to (I KNOW 6" when I see it, and when I don't, I've played enough to recognise it). And I didn't actually see him making the move! Then when he had killed it, he rolled 4" for consolidation and moved 4.5" into my Stealths. I tried showing him, and even asked another guy to back me up, but he moved it anyway. He won the whole game because of that.
Luck can't last a lifetime unless you die young.