There are tons of ways to lose BTC over time, scams like Trendon Shavers, Friedcat, Butterfly Labs; getting Zhou Tonged, Goxxed or Bitfinexed; more often than not, lost it in some ALT pump-and-dump or daytrading.  But quite often, simply because people can't afford to hodl and had to sell to paid expenses/debt or emergencies.  So it is actually quite difficult not to make wrong calls along the way that sees one's own BTC holding reduced substantially.  Fortunately, due to exponential rise in fiat terms, people normally end up with more, in fiat terms, than years ago.  But that still leaves a bitter taste for some as they know that they would have substantially more BTC had this and that not happened along the way.