What happens to a cat when it eats a lemon?

Most cats hate the smell of all citrus fruit (lemons, limes, Oranges, grapefruit). They will physically recoil from it (although, my uncle once had a cat that loved to eat oranges, so obviously there are exceptions). That is why most of those cat repellent sprays that you spray your sofa with are citrus-based.

If you are asking if it's dangerous for a cat to eat lemons, then...I really don't think so, but I can't say for sure. Apparently it's okay for them to eat oranges, so I would assume lemons are okay too.