Carrying cups and mugs is what I am absolutely worst at. I can't drink from a cup with a lid on it while walking... I've had people tell me that they didn't realize it was possible to spill things from a cup with a lid on it such that it was on your nose. Simply carrying the cup of tea with the lid on it I manage to spill even when its under halfway full sometimes. I've definitely spilled a mug that was less than 1/8th full, by not having realized how fast I was moving.
My boyfriend is addicted to coffee. He prefers coffee when I make it (he thinks its that I'm more consistent about things like how much coffee goes in and how finely its ground and such). However, his coffee mug is the absolute worst mug I've had for spilling things with. I can't carry it across a room, while concentrating on not spilling it, and have any luck. I've gotten to making coffee and then making him go pick it up himself, because I've spilled coffee on things hard to clean it off of too many times.
Mugs are in fact, incredibly challenging. I'm fine on a balance beam, but I can't carry a mug of coffee even a few feet without spilling it.