Not sure if anyone has talked about this before, but I just had an idea for a great business opportunity for big miners.

If you're a miner, you want competing miners to mine invalid blocks that will be orphaned, because that means there is less competition for rewards on the "real" chain. Your own profits are determined by the share of your hash power relative to all hash power that isn't wasted on orphaned blocks. Therefore, if you can trick competitors into consistently wasting time on invalid blocks, you eventually make more money, both due to being able to snipe high-fee transactions, as well as a higher number of block rewards once the difficulty resets.

Luckily, the soft fork consensus mechanism provides an opportunity to do precisely this. Say that there is a soft fork proposal, BS, which will take effect when 95% of blocks mined support it. Further suppose, that you control 50% of hash power, and that 45% of the hash power has already switched to mining BS.

If you pretend to support BS by using the old software but modifying it to set the flag, then the fork will activate, but with only 45% of the hashpower truly supporting it, while the other 55% supports the unforked version.

This means that as soon as a transaction happens which isn't valid under the BS rules, 45% of the miners will effectively be wasting their time on a chain that will eventually be orphaned, meaning more profit for anyone who doesn't enforce the rules.

Of course, you still have all the usual issues with forks, and people might switch to the 45% fork, even though it has fewer blocks, so the attack is likely to not be reliable in practice.