Another thought we had was that the pan and lid might contract (and expand) at different rates when heated and cooled. But if this were the case, it seems like cooling the pan would have solved the problem. We're really at a loss with this one.
Besides the tragedy of losing his tasty breakfast, it's looking like our friend is also going to need to replace his pan and lid!
Ever had something like this happen to you? Any ideas for getting the lid un-stuck?
UPDATE! We got in touch with a representative from Calphalon to see if they'd ever heard of this phenomenon, and it turns out to be more common than we thought! Those of you who said that a vacuum is created between the pan and the lid are totally right. Calphalon says that this can actually happen between any pan and lid of any material, not just nonstick and stainless steel. They don't have a recommended method for un-sticking the lid, but do mention that the pressure will sometimes release on its own over time.
Related: FAIL! Silicone Baking Liners
(Images with permission of Adam Hilliker)