Flying across Europe: on a hot and sunny day with enough moisture in the atmosphere the upward movement of air can result in these spectacular formations. These Cumulonimbus clouds rise up to tens of thousands of feet (this one was +40,000 feet) tempting aircraft into flying just too close. Our weather radar showed solid red with magenta flecks indicating severe turbulence, icing and probably hail. That would definitely spill the coffee.
You can also see another aircraft running from right to left who was being much more adventurous than us in trying to fly between two cells. We went around the whole lot.