Cateye Strada wireless came off once. Heard it... wasn't sure what it was... found out what I lost about mile and half later. Loop back, grabbed it on the bike path, rushed to train barely to make it in time.
Dropping on to me on the other hand most recently was 2 inch thick tree branch on my head... glad my helmet covered me nicely. I was wondering what were the chances of that happening as well as it landing on my helmet not on my back, hand, etc... (Road bike so I stretch out quite a bit)