I didn't mean to cook them really. I'm a cook in a vegetarian restaurant (yeah I know, that's a great start for a story about cooked slugs) and last year, my coworker was ill, I was alone with way too much to prepare for noon, I had to cook some spinach (a lot of spinach !) so I left the leaves to soak in cold water with a bit of alcohool vinegar to get all the bugs and stuff off, and rinced them. Didn't have any time to check further, and didn't worry cuz it usually does the trick, it's only when cooking them that I noticed some of the onion slices in the pan had little horns. Took me a while to realize they were melted slugs (they really melt and become all flat !). I removed everything I could find but my guess would be I didn't get them all. It all went into the blender afterwards so it was absolutely unnoticeable I guess. Yes it got served. No I didn't have time to prepare it again from scratch (and had no spinach left anyways), and no nobody got sick. It was just kept secret. Nobody had to know.