I've had bad spectators before, I don't count hecklers in that group. When I say bad spectators I usually mean someone that is uninterested but agreed to see what's up anyway. They don't really want to watch so they don't pay attention. I had this one spectator that spent the whole time I was performing staring at a girls ass so far to my left that I wasn't in his peripheral vision anymore, and then he told me he knew how the effect worked. I'd be surprised if could even describe the effect.
But he wasn't the worst. Last week I attended a friends birthday party. We were out on the patio and I had just finished giving my friend the gift of magic for her birthday. Her neighbor came by walking his dog (apartment complex) and as I already had my cards out I went to show him some magic. So the first bit of annoying was he called the effect a joke and immediately showed me the card I had him take. He said "I forget how this joke goes." I though maybe he was talking about the old gag where you fan the cards facing you and have someone take one and you tell what it is. So I laughed and played it off. I started over and he said it was a joke a few more times before I realized he did not know what was going on. I explained to him that it was a magic effect. And started over I had him take a card and I had him show the birthday girl for validation later. He kept glancing at her the whole time like checking for permission to interact with me, she was kind of ignoring him and I was too busy trying to do magic to read into it. I was doing the Biddle effect (for those don't know the Biddle effect is a magician in trouble plot where the magician doesn't find the card and has to recover through magic) and I turned up the indifferent card and acted like it was his like I'm supposed to, and instead of correcting me he confirmed that it was his card. I was flummoxed, I knew it wasn't but everything about him said he believed it to be his card, his tone, his body language, so either he was an very good liar or truly didn't realize that it wasn't. The only other person who knew what his card was projecting sod off at him with her body language so I wasn't about to ask her. I slid into a few phases of my ACR so that there was at least some magic happening and then bid him farewell. I later inquired about him to the birthday girl and it seems that he has a huge crush on her and she does her best to ignore him so as not to encourage him. So far he was the worst spectator ever.
*Update: Recently had a new worst.*