First notice: While playing with my friends on a skygen, I noticed that when I move my character's head down the windows taskbar would show up, even though I didn't have any UI's open such as the pause menu or player inventory, just for clearification I had (and still have) my Minecraft's fullscreen disabled.

Summery: Even if you don't have a game UI open, when moving you're character's head down, you can accidently open the Taskbar potentially causing accidents while playing.