When you have an unmoved client, but a spigot/bukkit server, any header/footer on the tab list stays there while connecting to a completely different server (e.g. my home-run server to hivemc.eu). If you look at bug copy.png, you can see that it says OneWayViperZone, which is my server, but it says brohoof which is the name of the server I am currently on.
It disappears after I restart Minecraft (not the launcher).
Reproduction:
1: Go onto a server that has CustomTab installed and active. Observe the header and/or footer. (It is darker and doesn't have a face and/or connection bars)
2: Then, go on a server without CustomTab (or a similar plugin) and note that the exact same header/footer is visible.
NOTE:
It is modded, because I play with mods, but it is also seen on vanilla minecraft.