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.
Many people do die in plane crashes due to human error, it happens quite a lot...
Same thing goes for surgeries in life or death situations. It can happen, but the % for a forced error is very small.
But still, it happens...
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.
I didn't say necessary in a lifetime, but the math is actually quite simple, there is a fixed amount, and from that fixed amount, bitcoins WILL be lost. Eventually, they must seize to exist.