Sometimes when you reload a world, you will die at the world's spawn point. If you try to respawn, only sometimes does it spawn you in your bed, but more often above it. This will happen even if you had full health and there were no threats nearby, or even if you weren't at the world spawn. It's extremely inconsistent and can/will hinder your experience of the game. 
For example; I've been leveling up enough to enchant things. I'm one or two levels away and as soon as I'm back in the world, I've lost more than half that XP, along with my items. The items aren't too big an issue, as they all drop, but it can be if your spawn is near a body of lava. It hasn't happened just once either. It happens almost one-third to one-half of the time I reload the world.