Ever wondered how supermarket baggers manage to totally ruin your groceries. They have obviously studied for this degree: Earn a degree in grocery mangling.
I should offer this course, I'm sure I can add some science to work out the precise weight of overfilling grocery bags based not only on the strength of the plastic, but also the strength of the customer. The older and frailer, obviously the fuller and heavier the bags, but there must be a correlation to give the optimum bag weight to customer strength. You could also study the science of optimal bagging including the effect of putting half gallon of milk with half gallon of orange juice in one bag. The physics and chemistry of this and its affect on bag strength, and the biological affects on the human conveyer could be a new research area.
It was once suggested to me that the change from paper bags carried on the hips to plastic bags carried in the hands was the cause of increasing back problems in the US. Perhaps I could even get NIH funding.