Sorry if this has been found and mentioned before, but:
If you start a practice or qual (i.e. from pits) and get on the gas right away in a faster car, with the limiter off, you can break the speed limit with no penalty--even upon reentering the pit lane--the speed limit is effectively removed for the remainder of the session.
I've been able to reproduce this in the last few versions, on any track, but it only happens if you accelerate past the speed limit in the first couple seconds of play (most of the cars aren't even fast enough).
This only works in practice/qual.