Because someone tried to upvote you on a smartphone, and Hacker News has a stubbornly non-mobile friendly design, with no undo.

Does anyone know what the rationale for not allowing users to undo votes is? I'd be happy even if it were only accessible for the first few minutes after the initial action...

There have been several times where I've accidentally upvoted low-quality (sometimes malicious) posts, and I never feel good about it.

Lol. Yep. I accidentally do that every so often, and I always feel bad about it.

Individual downvotes are sometimes errors and are usually quickly corrected.