So, this might not be the dumbest thing I see non-knife people do, but it might be the most consistently dumb thing...
Way too often when I hand someone a knife, the first thing they do with it is either brandish it like it's a katana or a rapier or something (often accompanied by some vocalization reminiscent of Bruce Lee), or they pretend to cut something (or someone) that shouldn't be cut...or both. It's made extra scary (for my safety and the safety of others), because the way they hold it immediately communicates that they have very little experience using a knife. I sometimes feel like I'm the only person who doesn't really think of my knives as weapons. It's as if the simple act of laying their hands on a knife is enough to trigger some sort of violent outburst from them (even if in jest, but Freud said there are no jokes). If that's how non-knife people see knives, it makes me start to understand where the fear/stigma surrounding knife EDC originates. Dang, now I'm sad.
In short, though, I've learned over the years that if someone asks me for a knife, I ask, "what do you need it for?" Then, if it's an appropriate use for a knife, I perform the cutting task. Saves me a lot of "OH NO!"