Post-study work visa (employer assisted)

It states on mine that I may only work for my current employer.

Does anybody know the penalties that apply if this condition is violated?
Specifically, I continued to do 3-4 hours of extra work for my previous employer while still on my main job but forgot about this restriction (my previous visa did not have such restrictions, but I'm aware it's not an excuse....)

Would INZ take any action?

Thanks.