A few years ago I played in a simul against a strong FIDE master. (I think he's an International Master now.) We played into an opening which I knew fairly well, so for about 9 or 10 moves I was doing quite well. It was a closed position, and most of the material was still on the board so I guess it was fairly complicated at that point. (Sorry, I no longer recall what opening had been played, but he opened as White with d4.)
On one of his turns the master quickly reached out for a piece, then pulled his hand back and went into a fairly long think. On the next couple of turns he also stood for a long time at my board before moving. At this point a large crowd had gathered around me, thinking that something special was happening. I had no idea what was going on, and didn't understand why he was taking so long at my board.
The next time he came around to my board he stopped and spoke to me, saying something like "What I was taking so much time over was trying to calculate whether I had mate in 6 or mate in 7. It's now mate in four." Then he played a check and waited for my reply. The crowd laughed with delight, myself included, and watched as he finished me off. That was great fun!