Minecraft 1.17.1
Server End.
Vulcan Anticheat.
Players getting banned while using the latest version of litematica. I have not personally tested it yet but will soon.
the anticheat is throwing up a bad packets issue more specifically "Post HeldItemSlot packets" and it is auto banning players.
if you could help us sort out this issue that would be most helpful. Please reach out to me if there is any other information you need

I'm assuming it's then related to either the pick block feature (using the pickBlockLast hotkey) or the Easy Place mode, which also does automatic pick blocking before placing a block. Nothing else in Litematica should be switching held items. So test those first, I guess.

As to what can be done about this... what exactly is the plugin checking, or what is the mismatch it sees? Is it simply timing related (item switch and use action too quickly in succession), or does the inventory state somehow not match what it's expecting?

There are pick block (and Tweakeroo's hand restock feature) related glitches in the mods that I have yet to properly look into why and how they happen. My best guess is that it's either related to server to client inventory sync delays and thus inconsistent inventory states when the client mod swaps some slots very quickly, or it's some kind of issue with bad packet order in relation to sending a use packet after switching the selected hotbar slot or swapping the items in some slots.

still an issue as of today.
With the Grim Anticheat (and very likely others), the printer fails "Post" (so post motion update/wrong packet order) with every block it places. And, sometimes, "FarPlace" and "PositionPlace".

I linked the relevant checks