The other day at work I was walking somehwere carrying a box full of important papers. I wanted to take my coffee with me, so I dropped my mug in the box with the papers . . . and it was totally fine. I don't even think the mug was standing up.
Needless to say, I'm quite impressed with this mug. So if you're looking for a good travel mug . . . this is by far the best that I've seen!