The reason the tree octopus as described by that page seems obviously, totally fake to me is the absolutely janky "photo". Let's count the issues:
1) obviously photoshopped -- a real octopus on a tree branch would look totally different, it would sag in some places, it would affect the pine bristles underneath, it wouldn't have a shadow that makes it look like it's hovering an inch above the branch, etc. Also, that octopus image looks totally out of proportion, but I can't pin down why -- I _think_ it's because the level of detail is higher than for the branches.
2) It looks exactly like a regular octopus. Not only should an animal the size of a small bird have different proportions from a regular octopus (compare e.g. bats and fruit bats, or cats and tigers), but it should also look only distantly related to a regular octopus because it's adapted to a totally different biome.
All that leads me to the following conclusion: Common sense, in the sense of broadly understanding how the world works, really is what prevents you from getting fooled, and the more things you understand, the less likely you are to get fooled. Also, the more information a hoax has, the more likely it is to get exposed, because just one sufficiently glaring inconsistency can sink it.