According to a new study, You are able judge how objects will fall when you are upright then when you are on your side. The study took 15 volunteers with laptops and showed computer generated pictures of vases positioned at different angles at the edge of a table and asked which way they'd fall. They were then tested both upright and lying down. The researches found that people were able to judge the fall better when they were upright then when lying down, presumably because we spend most of our time upright and are in our equilibrium.
And our senses do play tricks on us and since our brain perceives gravity's direction through multiple senses, our perception of gravity can be a little well... Off. Stand up hold your right arm out to you side and stand on your right leg with yours eyes open, now do the same with your eyes closed and see how by lacking that extra sensory information, your balance goes off and your brain has tricked you.