I could definitely imagine a company like Mt. Gox, who is holding a lot of Bitcoins on account for people, accidentally locking themselves out of one of their reserve cold storage wallets (or having one wallet stolen by an employee). At that instant, they have fractional reserves. But they could easily set aside a small portion of profits each day to re-fund that wallet.
Which would be worse? Announcing to the world, "We locked ourselves out of one of our wallets, but rest assured...." (potentially causing a run that puts them out of business), or to quietly build the reserve back up?
Pretty much this has already happened to one business (or that is his claim). I forget the name off the top of my head. His action was to suspend all accounts. The last I heard he's intending to pay everyone back but it's hard to see how, it was a lot of bitcoins lost.