That was very common years ago when the price of bitcoin was low, people bought bitcoin and they did not thought much about it and they forgot about their coins but some time later they realized the price of bitcoin had gone up dramatically and they wanted to get those coins back but since they did not took enough precautions to recover those coins they lost access to them, however I believe such scenario is going to become less common as the value and the potential of bitcoin as a currency becomes more widespread to the general public.
It's still common for me actually to be honest;) I used to have so many accounts on different faucets and casinos, and I would even deposit in almost every place to gamble, and then many of them would have balances too small to do anything at all (too small for min withdrawal) so I end up forgetting about them. Probably if I lumped everything together especially after price boom I would have over hundreds of dollars.