Do you mean by claiming that the block containing it is invalid when it is valid, or visa versa? Or something else?
Also, how can there be a fee to trying to send an invalid transaction? Such a fee would have to be sent as part of a transaction, yes? Isn’t it just that someone spamming the network gets dropped from the gossip protocol stuff?
The thing here is that the validators don't need to be honest when responding to a slasher report.