I had an undergraduate TA for my Physics 221 class at ISU. Not only was she an undergrad, but she was an ........ (shudder) engineering student. This was the spring of '87.
She was forced out about a month before finals, and replaced with a guy who spoke English about half a degree better than he spoke the language of birds. Word on the street was that all the Physics grad students sabotaged her nonstop, and it became necessary for her to stand down just to prevent a general rebellion in that department.
Too bad, because she was really good at what she did. I can see an objection to having a non-physics student be a TA in a 300 or 400 level class, but the introductory weed-out class? I wonder if their main objection was "she's helping those kids learn the material too damn well, and we're not going to be able to weed out ANY of her kids"?