You've seen paper wallets and stuff right? The reason these work and still retain their value is because they have the private key stored on the block chain, if somebody tried this everybody would be able to tell because they'd get some of the notes, check the blockchain and then notice that the private keys etc. are fake.
Counterfeit Bitcoins are an extremely unlikely scenario right now, we've already had exchanges try to fake their trade volume and it was spotted very quickly.
After noticing that my deposit had arrived ...
Status: 26 confirmations
Date: 21/01/2014 15:13
To: cryptsy 4JF3YMUj8yxMZUayN1ohfN4nvCuGMRSQjn
Debit: -0.00218592 42
Transaction fee: -0.00000001 42
Net amount: -0.00218593 42
Transaction ID: e76f68b1984f2a732cdadcc5dd2acb1a8d4c42736b6f3bd390daa03599835ea6
I stepped out for a smoke, then came back to this.

... I mean

(edit) Seconds before, the deposit's balance was showing as available on the order page.

Then this on the order page ...


If I'm seeing a surplus, no doubt others are missing coins.
Tread lightly folks...
UPDATE
The 0.00655776 42 is still there.
UPDATE II
The website is down.