Martha once tried to sell Martin's Star Wars merchandise to Comic Book Guy for extremely cheap. She was warned by Bart Simpson and Milhouse Van Houten before she sold them and she decided to not to sell it. It was also hinted by Martin that she shoplifts a lot, "stuff she doesn't even need".
She also was responsible for all the kids being sick at Martin's already lame birthday party by serving diseased oysters instead of cake. With the exception of Bart (who fed it to the cat), Lisa (who was taken off in a stretcher just because she wanted to leave), and Seymour Skinner and Edna Krabappel (who were making out in Martin's girly playhouse), all the kids were taken away in ambulances.