Hi,
What happens if there's a fork really deep?
That's astronomicly unlikely to occur, but even if it did, all the clients have benchmarking hard coded into them that prevents them from going back and reorging beyond their latest benchmarked block. The benchmarking basicly involves a list of block numbers chosen largely at random, and their established hash values. If there is any kind of blockchain attack that tries to convince one or more clients that there is a differnet true chain, it must also be able to match hashs with these benchmarks as far back as they go, and not all of the clients have the same benchmarking list, so even diving into your own client to find the list to match won't necessarily mean that an attacker won't run into a client with a different benchmarked list.