Yes, but I've been conditioned into thinking a fault with the vending machine is much more likely than a counterfeit coin. Just the other day, I was using a vending machine that had a special panel on the front of it, with the specific purpose of rubbing your coins on it in the event that the machine didn't recognise them!

I've definitely noticed worn coins before. Some of the current pound coins have been in circulation for 34 years, though, so I'm not surprised there'd be quite a range of wear.