One of the more serious mistakes a בעל קריאה can make when reading the מגילה is actually not in the מגילה itself. This hit me one year when I heard this mistake made and unfortunately, did not have the reflexes to correct it on the spot. After we finish the מגילה we make the ברכה of הרב את ריבנו, He who fights our fights. The proper pronunciation is, of course, הָרָב. But when mispronounced הָרַב, I believe that means "He who increases our fights." (Or, Elie suggests in the comments that perhaps it might have a different meaning.) That's not a good thing at all. Honestly, if I hear this again, I would try to correct it ... unless someone thinks otherwise.