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.