After leaving and rejoining a world while fighting the ender dragon, her health and position reset to their initial values. This is very, VERY, annoying, especially if you misclick on the leave game button.

HOW TO REPRODUCE:

-Create a world in singleplayer
-Go to the end
-Hurt the ender dragon
-Leave and reopen the world
-Observe how the dragon seems to reset

(Here's a link to a video of what I mean)