That's kind of the point, you can back write a block chain with enough hash power. Just go back a few hundred blocks in the chain, and "fork" into an alternate

That's kind of the point, you can back write a block chain with enough hash power. Just go back a few hundred blocks in the chain, and "fork" into an alternate chain. If your chain can catch up and become longer, you win. This happens on a micro scale every day, when two people "solve" the next block at the same time. Inevitably, one of those forked chains gets longer and the other one is abandoned or "orphaned".