Fractional Reserve Bitcoin

What percentage, if any, do you think central exchanges could practice fractional reserve on bitcoins? (or any coin)

While all coins are basically transparent on the blockchain, it seems probable that the internal trade books of an exchange could be obfuscated to their advantage. Such as a coin "borrowed" by a leverage trader could still appear in a HODLers account, effectively making a new coin temporarily.

The only real coins in this case are the ones that a customer wants to withdraw. The same that happened in the vaults of the ancient goldsmiths lending out more certificates than they had gold. No one is the wiser until a mass withdrawal.

If something like this does happen, then it invalidates the "21 million finite cap", as untold amounts of bitcoins could be issued in this way to the profit of the exchanges, basically made from counterfeit bitcoins.

What do you think? Is it possible, and if so to what degree?