Do MIL's ever fully trust son-in-laws?
Electrical connections done with screws and screw-clamps should be tightened with the proper torquing tool. How often that happens, can't say. Heat does make almost everything expand. The connections aren't perfect, so they will have some slight resistance across them. Proper clamping force helps immensely, and there's enough tension for it to remain tight. The spring loaded connections used in a receptacle and some plugs is affected by the heat, too. The higher the current drawn, the more potential heat can be generated. Over time, and from repeated insertion/removals, that can loosen the connection and result in more heat, and sometimes, rapidly degenerating situations.
When I was in the Army, in one of our test stations, I happened to find the specs on one plug we used on our electrical meter. It was rated for ONE insertion...any time after that, the spring tension may no longer maintain specs, and the readings you might get from the attached probes might be suspect. Now, that was a real pain! Home plugs and receptacles are much more forgiving, but they can exhibit similar problems.
Bottom line, if the plug is easy to insert, it's time to replace the receptacle before you start to have real problems. Before I replaced all of them in my mother's house, if the cord was heavy, it might just fall out on its own if there was the slightest pull on it...not a good situation at all! I hadn't lived there for decades and only got there maybe once a year for a short visit, so didn't come across this earlier.