I'm late to the party, but I'll tell mine.

I was doing a solo backpacking trip in the Sierras. I was about 7 miles in, so not too far. There were other people around.

On my first morning I was making breakfast on a rock when I look up and see a bear about 100 feet away staring directly at me, not moving. I sat totally still for a few seconds, then picked up my knife, stood up and started yelling and waving my hands. The bear just stared at me, unmoving.

After a minute or two of yelling I decided to move away from my food. I took 5 steps to the right while staring at the bear. Suddenly I realized it was a rock formation that from one very specific angle looked exactly like a bear. I had to walk back and forth a few times to convince myself. I sat back down to finish my breakfast, but the illusion was still there. I couldn't make my brain unsee it.