Last year, my wife learned how to ride a bike, and other than a few ungraceful dismounts and not quite getting out of a pedal soon enough once, she hasn't had a crash.
That changed this weekend, when in a lapse of judgement, we went riding with a friend who thinks the biking laws are stupid. We thought we'd be safe as we were in a bike trail, so didn't have to worry about keeping him from salmoning, etc. But then he decides to grab her handlebar as she's riding. She said that she felt herself leaning, like he was (probably without realizing it), tilting her bike his way. She overcompensated, and they both went down. Other than a few bruises and soreness, she's fine, and fortunately her brand new 7.5 is as well, but he had a $70 repair on his Globe when he badly bent his front wheel. Of course he said she could have kept balanced even with him grabbing the middle of her bar, but he knows she's still not an expert in handling.
I guess you really never know where the hazards will come from.