The blockchain's motto is to Trust but Verify. Each node follows a set of rules called consensus rules , it is a set of rules that define what kind of a block is valid in the current blockchain.
Assume that a bad actor wins the POW competition and creates an invalid block. It then broadcasts this block to all the neighbouring nodes who are still trying to solve the problem because they don't know it has been solved. Now here's where the situation diverges into two scenarios.
- The receiving block rejects the block : Remember that each node has a local copy of the consensus rules embedded in their node's code. So it simply rejects the bad block and continues to solve the problem ignoring the bad block.
- The receiving block accepts the block : To accept the block, the node's consensus rules must be different from what define a valid block. Hence, the accepting node can change it's own rules and deem the block valid and pass the block to next neighbouring nodes. If many of the nodes do the same changing of their own rules and passing on, the valid network's security degrades and a fork is formed.
Now the forks case has less chance to happen if just a single miner is a bad actor. On the incentive of getting the block reward, a good miner will reject the bad block and rush to solve the problem as they still have the chance to get rewarded.
But if the intention itself among the miners is to define/change the rules and change what block is valid ,it's a fork and the example for that is the Ethereum Classic chain.