language help

[inhumandecency]

My grammatical intuition, which very rarely steers me wrong, is telling me that "sit" can sometimes be used as a past tense, in places where "sat" would also be appropriate, as in "he climbed up on the whale and sit down." Yet that also looks like it should be blatantly incorrect. Can anyone tell me if there's anything to this? The only alternate explanation I can think of is that something in my brain is making the common sit/set mixup, since "set" does serve as both a present and a past tense.

Update: Okay, okay, I'm wrong. I'm convinced now, so there's no need to comment unless you have additional thoughts on the matter.