Watch a sloth just nonchalantly (and slowly) walk past a giant anaconda, even touching it and passing over its head. Either the sloth knows he's in no danger, or he's just plain dumb. The snake could have easily grabbed the sloth and snuffed its life out, but for some reason, it didn't. The discussion at reddit gives us some clues.
1. A sloth doesn't make a great meal. They don't have much meat, and it is "slimy, chewy and gamey."
2. Sloth fur is covered in algae, which is an ecosystem for insects, fungi, parasites, and other creatures. The smell can easily turn a predator off.
3. Sloths are not on an anacondas list of favorite foods, but they will eat one if they're hungry enough. However, these snakes eat such large meals, they don't get hungry until they've digested their last meal, which can take days.
Sloths are slow and have poor eyesight, but considering that this one didn't even change direction after he touched the anaconda, we have to believe it's just dumb. And lucky.