I'm sure it has happened, but it is not a rampant problem. Linemen are probably more likely to get robbed by a crackhead than electrocuted from a backfeed. I saw a POCO do a demo about back feeding a 25 kVA transformer from a 5000 watt generator. The first try, the in-rush tripped the breaker on the generator. The second try killed the engine completely. They had to fiddle around with the governor to over-rev the engine while throwing the transformer on line just to get going. Given that and the fact that there will be many neighboring services being energized at the same time, I bet it happens very rarely.