This happens to me all the time. I use the teapot for moodlets when I'm skill building a sim and the action to grab another cup pops up in their queue before they're even done drinking the first one. If I don't immediately have them empty the pot before I queue up whatever skill building activity I want them to do, they will continuously cancel the skill activity in order to grab another cup of tea. I didn't even realize this was a bug because it's been happening for as long as I've been playing, thought it was just a tuning issue. (Edited my comment because apparently I wrote about two separate issues.)
