Fortunately false credits are not accepted anywhere so Mt.Gox will have trouble spending them all.
Seriously though, Possibility? Sure.
https://en.wikipedia.org/wiki/Bucket_shop_(stock_market). I don't really see the incentive for tampering either while their real business model is running so smoothly. The only way to know for sure, is to organise an everyone-withdraw-your-money day. But even that costs people money, which is partly why bitcoin was invented in the first place!
Take heart from the fact that market share is levelling out. Soon any tampering would be made much more apparent by looking at data across exchanges.
The biggest risk is still the hack + fake sell scheme that happened back in 2011, it would be worse now because there are so many other places for the attacker to buy bitcoin during the ensuing panic. *related:
http://youtu.be/E1xqSZy9_4I?t=1m2s.