This question already has an answer here:
One month ago I bought a set of stainless steel cookware from Cuisinart. Today I noticed that the smallest pan is not flat - the base has bended so that the center is a little lower than the edgees.
That made me wonder what could have caused it? I was quite watchful to wash it only after it cools down a bit (though I'm not sure how my girlfriend was). Can overheating cause that too? Or heating without oil? (my girlfriend uses it to toast toasted bread)