I'm pretty sure there should never be a time when this happens;
I'm leading an entire race on "Down the Drain" (The one that's in the little canal type thing under the bridge in the La Mesa area) by literally a mile, and proceed to hit a flat piece of cardboard on the track. This somehow thrusts my front end into the air causing me to do a wheelie, and when my front tires hit the ground again, I spin out completely, causing me to have zero chance of winning.
This seems to be the case a lot of the time when you race online. One person f*cks you over, and the rest of the race is pointless because you don't have a chance.
Seriously, though, what's with the f*cking cardboard on "Down the Drain?"