I was originally a biology major and only ended up finishing the minor in part because of this. We had a lab assignment to do a live vivisection of a limpet to be able to view some of the circulatory systems in play in a still-living creature. We were supposed to paralyze them first, but whatever I did was incorrect. That thing was writhing and squirming and trying to get away and showing all the outward physical signs of being in pain the entire time.

Limpets don't even have brains, just cerebral ganglia, so it's likely it isn't really a sentient creature that has any conscious awareness of what is happening to it, but man, that scarred me. It felt like I was torturing something horribly and I knew that was not the career for me. There is no way I'd have ever been able to do that to a mouse.