I might have crossed 20 hr this week by mistake

I was checking my number of hours worked this week and it is coming out to be 20 hours 30 mins. Because I just took s new position with USG (stripened position) and forgot to account for that.

What will happen ? Worst case
What can I do

Ask your school about this.

Check with your school but in a nutshell:
It’s no big deal, work 19h30 next week, it’ll all be averaged anyway over the semester. Basically, the key factor is that you work on campus (or an approved off campus university facility) and don’t go over 320h/semester.

That’s the first time I hear about the cumulative semester thing. The federal code of regulations states the 20 hour limit as a weekly limit, not a cumulative limit. But I don’t know how strictly that’s actually enforced. I can’t imagine that going over the limit once by 30 minutes will result in more than a slap on the wrist, but I am not in a position to give you a definitive answer.

I agree with aunt bea: if you are worried, go speak to the international student office at your school.