Pretty much. Or, to be more specific, a situation where the entire economy (pretty unlikely, but the problem would still be there with less than 100% adoption) is running on some relatively small part of the "original" 21m BTC (let's say 100,000) and somebody finds an old wallet containing 50,000 BTC from the olden days.
This would not only cause a huge mess if it happened, the very fact that it might happen could prevent Bitcoin from catching on.