If you have 2/3 of the validators, there is no way that you can be punished. You will effectively be able to produce the blocks and tell the network that they are valid.
If you don't have that, the most that you can do is to attack the network by proposing bad blocks to slow down block production. Anytime that a (selected) validator proposes a bad block, the other validators that catch your mistake will snitch on you (through attestations) and the validator will get their stake funds slashed. The more your funds are slashed, the less of a chance you will have to be proposing new blocks in the future.
If you don't have that, the most that you can do is to attack the network by proposing bad blocks to slow down block production. Anytime that a (selected) validator proposes a bad block, the other validators that catch your mistake will snitch on you (through attestations) and the validator will get their stake funds slashed. The more your funds are slashed, the less of a chance you will have to be proposing new blocks in the future.