The other witnesses would flag this block as a bad-block and reverse it

No, since RCs are not part of consensus, this would be a perfectly valid block and would be included on the blockchain just like any other block. It really wouldn't create much of a problem either, the block size is limited anyway so at worst some accounts would be able to get in some more transactions than they normally would have been able to in any blocks created by that witness - which is a function of the amount of witness votes they have.