When my wife was a teenager her father helped her buy her first bike - a cherry Honda 305 (yes, it was a number of years ago). Her father rode it home for her and before she could ride it herself her boyfriend announced that he would warm it up for her, got on it, blasted down the street, t-boned a car and totaled the bike. Needless to say, her approach to letting people ride her bikes is "you break it, you buy it".
The last time I borrowed a bike it was to teach a friend how to ride. It took him less than half an hour to go WFO, drop the clutch, and wheelie through the group of people watching him. No one got hurt, but I think my days of borrowing bikes, lending bikes, and/or teaching people to ride is drawing to a close.
Non omnes qui habent quadrigae sunt auriga. - Anon
I returned, and saw under the sun, that the race is not to the swift, nor the battle to the strong, neither yet bread to the wise, nor yet riches to men of understanding, nor yet favour to men of skill; but time and chance happeneth to them all.