Neither mistake is any less than the other--ignorance is no excuse,
particularly in a case where the person (OP in this case) can no longer
even claim ignorance.
It has indeed happened, not always from downed lines but supposedly
isolated lines. Our local REC had a lineman burned severely and very
lucky he wasn't killed by such an incident where the generator was put
onto the line _during_ the time they were working on a feeder that was
physically disconnected from the line source but a secondary transformer
was on the other side.