Nothing that's bad. "Worst" can mean good relatively, but it's confusing

It's a very slippery slope. Thankfully, we have to thank the network's design as to how hard it is to do so.

Bitcoin, the computer network, comes into existence when many people run bitcoin implementations following the same rules. This practically rules out any controversial changes, which could never get broad social consensus.

This makes Bitcoin resilient to disputed/bad changes but open to good, obvious incremental improvements.