How does Bitcoin solve people downloading the wrong chains? You could simulate the Bitcoin blockchain from genesis block with less forging power, the difficulty would therefore be lower and people would download the wrong one, right? Surely the current miner doesn't pass back all the info needed to build a chain from the genesis block every time he solves a hash, right?
You can trick a node into download a chain with less difficulty, but you can't trick the node into using it. The former is a potential resource wasting attack, the later is required to defraud nodes.
This attack is most disruptive against bootstrapping nodes (because the difficulty was low for a very long time) which is why many clients use checkpoints to limit the disruption a node can accomplish. It is important to understand that even without checkpoints a peer will still be able to determine the chain with the longest difficulty it just may take additional resources. Bitcoin nodes may be a little too trusting and they can be hardened against "disrputors" but I doubt it will ever become a useful attack. Even if a node is naive an entire chain of headers it is only 8MB per 100,000 blocks (two years of blockchain history) even modestly "smart" nodes can limit the scope significantly and thus put an upper bound on the resources a disruptive node can waste.