"I have a few of those Tupperware lids too. They are not dishwasher safe, hand wash only. And they can't be used in the freezer either. "
This has been my experience, people are telling me dishwasher safe, but mine are not! They warp, bend, melt, flex, and shrink. After that they no longer seal, or are a really tight fit.
From the Tupperware website itself: "Tupperware brand products purchased after September 3, 1979, can be safely washed in the dishwasher." Almost all of my tupperware, (and I have 3 cabinets overflowing) was made before that date.
It's possible that hard or soft water, water temperature, type of soap etc etc makes a difference.
The cereal keeper lids are the worst. I had one that shrank in the dishwasher a good half inch smaller, and it was noticably less flexible. I tried to force it to fit on there and it just split in half right down the center.