A failed challenge costs five points, not your turn

If you challenge a play and every word in it turns out to be good, the play stands and five points per word challenged are deducted from your own score — your opponent gains nothing — rather than your losing the turn. A play that forms three new words therefore costs fifteen. An invalid play still comes off the board and still costs the player who laid it their turn, exactly as usual. Losing a whole turn is heavy enough that nobody challenges a play they merely suspect, and heavy enough that laying a word you know is invalid becomes a live tactic; a points penalty keeps the deterrent without the swing.