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Although Pyrex is designed to go in and out of the oven, it still has to deal with thermal stress and fatigue. This can result in the casserole (or whatever dish you are using) breaking. When you take your casserole out of the oven, it's best to let it cool in a neutral-temperature area -- one that is not too much warmer or too much colder than the dish itself. For this reason, placing the casserole on top of the oven range is not a good idea -- nor is placing the dish in the refrigerator right away.
Last night I made a new recipe, cheesy mac casserole, I put it all together and then put it in a oblong pyrex glass casserole dish. The dish had been sitting on the counter since the day before so it wasn't too cold or too hot. As I would usually do, I preheated the oven and then put my casserole into the oven. Well, about 15 minutes later, I heard something in the kitchen, but just ignored it. I came to check on it just a couple minutes later, and the whole dish had broken all over my oven!!! My casserole was ruined and my oven the biggest mess you could imagine. Not sure if I want to purchase any pyrex again. Not a very happy consumer.