On many servers, especially mineplex, if you are on a mini game you can see spectators. I believe this is still an issue in later versions, but I know that it affects version between 1.8 and 1.12.
what I expected to happen: spectators are completely invisible
what actually happened: spectators briefly show up for about half a second
steps to recreate:
go on mineplex
go on block hunt
hider
pick a spot and play normally
look up and down really fast without moving
you will briefly see a player head as if it is a spectator
this is really hard to screenshot, so I probably can’t get proof in that sense easily.
Mineplex is a modded server; unless you can reproduce in a completely vanilla environment, this is not a bug in minecraft, but instead their modifications.