After a second player joins the world, all hell breaks loose.
Aggressive monsters sometimes lose their skins and walk around invisible while still able to hit the players.
Mobs in general lose all physics and jump around. Sometimes merging into ground halfway while able to still walk around.
Aggressive monsters sometimes get a speed a boost. Creepers instant explode without flashing.
When breaking blocks, the dropped block can appear blocks away if it does appear.
Players sometimes lock up and can't move. Their character can't be seen by other players on the world.
I do have video of most of these happening multiple times on new and older worlds.