My mug from YAPC::EU in Munich broke today. I poured water into it from the kettle, heard a cracking sound, and brown liquid started to appear from the base. It now has a fine curving crack all the way down one side.
Unlike some mugs, these ones don't seem to have infinite lifetime[*]. At my previous house the other housemates had three, and by the time we left two had died, of which at least one was due to similar non-impact induced failure.
* Oldest mug I have in continuous regular service is probably an "I was plundered from Jorvik" mug which dates from a family holiday in 1986ish