Im pretty sure you can also receive a ! (Dirty lap) indicator if you messed up on the last sector of the PREVIOUS LAP.
Yes that's very familiar to me from Forza 3, and also in Forza 5 now.
What annoys me, well that list is massive so I'll keep this limited to Forza, is other cars driving into you screws your lap up. Even at harder skill levels for the Drivatars they can screw you up.
Saying that, what seemed counter intuitive to me was the higher I set the Drivatar skill level the easier the game got for me (I'm still crap but not as crap), but I suppose that's because the Drivatars are less prone to stupid braking and other stuff.
All good, but a little frustrating, fun.