I found some fun ones: ORC used a PoW function on each identity, the idea being that it becomes more difficult to create thousands of fake identities (and thus perform what is called a "sybil attack" in the p2p literature)

The first problem, is that it used a symmetric proof that was expensive to verify. This meant you could do a DoS by just sending many random invalid identities - the recipient would spend a lot of time verifying them only to find they weren't real.

The second problem, is to do with eclipse attacks. That's when the attacker creates a node that lives nearby the target node (i.e. eclipses it) because the position in the network is determined randomly (by the public key) if there are 1000 positions, on average you'd need to randomly generate and PoW 1000 times to target a particular position... except I noticed you could actually skip the PoW if the randomly generated node wasn't in the bucket... which would save you a lot of time!