It’s the little invalid lap symbol up next to your lap time.
You remember how Forza 2 would just deduct time for faults? Yeah no, now they void the lap in terms of best lap. Like, say you ran a 5:35:08 on the nurburgring, but on the last turn you were behind another car and drafted, or let your tires get too far on the curb…lap doesn’t count, and it voids your next lap as well simply because you faulted in the last section.
RETARDED.