Sometimes, seemingly at random, the player's left hand will rise above the map and obscure a bit of the bottom left corner of the map. I have observed that this usually happens sometime after entering third-person view (F5) and returning to first person, but this does not always create this result and is not always the cause.
I have attached a screenshot of the problem.
Also pictured in the screenshot, this bug: https://bugs.mojang.com/browse/MC-53184
I'm not sure why the linked bug was removed, but it concerned the disappearing player blips on the map, discussed here: http://minecraft.gamepedia.com/Talk:Map#No_map_blip
You were on a boat when this happened. If in a minecart or boat and press F5 while in, your hand will lift up. So make sure you do not go to third person mode whilst in a boat/minecart. I've had that experience before and when I got off, I pressed F5 until back in normal and it went down.