Just two days ago I lost my rear light in traffic. Didn't bother getting it back.

Twice I've had a pannier fall off (one a over pothole and the other while hopping a curb). Luckily it wasn't in traffic and the contents were undamaged.

Over the years I've had the following fall off (or dropped) from my bike while riding: a few lights, a few reflectors (usually the ones on pedals), a sports watch (I had attached to my handle bars), a couple water bottles, and a pannier bungee cord bracket.