In Article 4 of the WCA Regulations, it doesn't mention anything about what the scrambler does if he/she messes up during a scramble. Some situations in which this may happen is:
1.) If the puzzle pops while scrambling, and he/she loses his/her place.
2.) If he/she gets distracted while scrambling and mistakes R' for R (or R for R', etc)
I know that scrambler generators used for official competitions also returns a picture of what the puzzle would look like after doing the scrambling sequence. So, if the scrambler finished the scramble and finds that the resulting puzzle doesn't match the one in the picture, what will he/she do?
If the puzzle is something small like a 2x2 or 3x3, he/she can just solve the puzzle and redo the scramble. If it's something big like a 4x4 or 5x5 and it takes time to solve, that will delay the scrambling time and, by extension, the round.
This probably doesn't happen too often, but it does happen, which results in different scrambles for some people (which may or may not give them an unfair advantage)
EDIT: woops I didn't realize there was a thread for scrambling already, sorry ]: