I have not had any ammo to ignite from dropping. I remember many years ago (circa 1960 AD) the older boys (7th & 8th graders) throwing .22's against the gym wall popping them. I have seen .22 LR thrown in a fire, they popped like small firecrackers.
This doesn't answer the OP's question but when I was young and stupid(er) I used to cut 12ga shells open, empty the shot and powder, then shoot the center cap with my BB gun. From about 20 ft. I was a damn good shot with that Daisy 880. :-)
It is extremely unlikely, but stranger things have happened. The result would not be very dramatic and serious injury would be unlikely.
SAAMI produced a very good video for firefighers related to ammunition and components, you can preview it here. SAAMI
The relevance is that even if a dropped round goes off, without being captive in the chamber to build pressure and connected to the shooter to 'push off from', the case and bullet MAY separate, carrying only their individual mass. The case, being lighter than the bullet in most cases, is the component that may take flight. Another outcome is the primer just comes out of the loaded case because the breach isn't there to hold it in the case. None of it worrisome.