I was recently in Riverwood, casting Detect Dead following a Dragon attack to see if anyone had died.
Frodnar saw me casting the spell, and asked me if I could make him invisible.
I was given two speech options, either say no, I didn't know the spell (which at the time I didn't), or to say, "Poof, you're invisible!".
Since I hadn't saved in a while and was fearful of the consequences of making him believe he was invisible, I told him no. He was disappointed, and said something along the lines of, "Oh well, I'll have to think up something else then".
I'm wondering if anyone has encountered this dialog and knows what Frodnar does if you make him think he's invisible.