I have an ICC report from an accident dated October 5, 1950. Engine 5454 had an eastbound freight of 76 cars and a caboose. They derailed at the 37th car, near Erie, Pa. Although they sent out a flagman from the engine, they weren't certain whether a derailment had actually occurred - all they knew for certain, was that the train had gone into emergency (it was during the middle of the night). The 5454 was well-equipped with two cross-compound pumps, and was able to restore full trainline pressure quickly, even though the train had parted. So, they (incorrectly) assumed that the train had not derailed after all. They recalled the flag from the front of a train, just in time for a westbound passenger train to pass them, and run into the derailment. Nobody was killed, however.
JR