Well there are some cases of lost bitcoin, most of them happened in the past when the value was lower, and people didn't really paid to much attention to them. I don't say it will not happen again, but it will be insignificant. Yes human errors happen, but they tend to decrease as the "value of the act" increases. Take a look to air control in airports. The % of human error there is incredibly small. If it wasn't many people would die. Same thing goes for surgeries in life or death situations. It can happen, but the % for a forced error is very small. Same thing will be with bitcoins, the value is so high, that people will treat it with extremely caution, and the % of lost bitcoins for unforced errors will be minimal, so what you are saying can't really happen in a lifetime.