We really shouldn't have to go over "Bitcoin 101" on this mailing list, and this discussion should move to the not-yet-created more general discussion list. I started this thread as a sanity check on myself, because I keep seeing smart people saying that two chains could persist for more than a few days after a hard fork, and I still don't see how that would possibly work.
So: "fraud" would be 51% miners sending you bitcoin in exchange for something of value, you wait for confirmations and send them that something of value, and then the 51% reverses the transaction.
Running a full node doesn't help.