Affects Version/s: Minecraft 1.12.1, Minecraft 1.12.2
Fix Version/s: None
Step one: Create a floor of slabs of any type
Step two: Spawn a skeleton, make sure it doesn't burn
Step three: Set your gamemode to survival
Step four: Be amazed as the skeleton reaches unnatural speeds running over to you, making it almost impossible to hit.
Probably happens in other versions too, tested on these.