Bitcoin core will reject and transaction with an "absurdly high fee". This means that it will not accept it to the mempool and will not relay it. I don't remember what the exact threshold for this is, but I think it is actually fairly low.
Aah, I ran into fRejectAbsurdFee while looking into it. I have yet to figure out the criteria for an absurd fee but I'll post it here when/if I do.