Martial arts training camp. Average sleep time was around 4 hours per day, also, guard shifts round the day, so sometimes it ended up being 2. So towards the end of the week I was quite... sleepy. And this seems to have an interesting effect on visual pattern recognition.

One day, me and another guy were standing guard, around 4 in the morning, the sun was just about to come up. Making circles around the countryside weekend house we were staying in, I noticed that some people appeared with a truck and started to pick grapes from the nearby field. I promptly went and reported it to the other guy, so I was pretty sure of this observation, until I went back, and...

the truck and the people somehow turned into grapes and new people appeared to pick them.

Later that week I actually made up a rule saying "the guy standing in front of the house is, regardless of how much he seems to move around, a tree". Since I actually went there once and checked previously. Science over unreliable visual cortices...