At summer camp. A buddy and I are walking down a trail from our campsite to the mess hall for breakfast. There's a sudden rustling of bushes, and a very large, very low-to-the-ground, very feline shape runs away from us from about two feet away.
We look at each other, confirm that what we both saw was, in fact, a cougar, and ran screaming.
After our heart attacks we came to the conclusion that it was a small one, probably still quite young, and was likely more scared of us than we were of it, but still, frickin' cougar!