Minecraft Version:
1.12.2 Forge-14.23-5.2796
Inventory Tweaks Version:
1.63

While in spectator mode:
if you use the options to sort the chest it temporarily puts items in your hand/inventory. By spamming the sorting, you can get items from the chest and keep it.

https://github.com/FTBTeam/FTB-Presents-Stoneblock-2/issues/45

As far as I can tell it's already making sure the player isn't a spectator before commiting any actions on the server side: https://github.com/Inventory-Tweaks/inventory-tweaks/blob/develop/src/main/java/invtweaks/network/packets/ITPacketClick.java#L49

Sadly the items to get removed from the chest. The sorting does get send to the server too. You can see the items moving in there if a visitor uses the buttons.

The visitor had the items in his inventory, they were gone from the chest and he could give them back to me in the spawn area.

Oh, re-reading timing this fix wasn't in the current release (It is however, literally the first 1.64-dev after 1.63). Duplicate #525

Are the 1.64 stable enough for being in a pack tho?

https://minecraft.curseforge.com/projects/inventory-tweaks/files/2514356

This one is just 1.63 + the spectator stuff basically, so it... should be alright?

And it's not like I really heavily test even the release versions myself anyway.