For one reason or another, I had a hard time pouring drinks when I was a kid. I can remember a constant problem with me spilling stuff. A lot.
Problem was that if I were using paper cups or plastic cups or something similar, the lack of weight and stability would lead to me just knocking it over instead of filling it up. I'd try to pour some soda or something into it, but instead of the stream of soda hitting the bottom of the cup and filling it up, it would hit the opposite side of the cup and push it over - spilling soda (or whatever) all over the counter.
I can remember this happening on MANY occasions... and really don't even want to know how many times it happened that I don't remember. I suppose it never occurred to me as a kid that I was doing it wrong. Of course, eventually I started to realize my mistake, but it probably took a lot longer than it should have.