When moving items inside an Anvil this get this put into chat
InvTweaks: [ERROR] [6] Failed to trigger shortcut: toIndux = 45 (l1004 in ArrayList.java)

Can confirm this issue. Happens with the latest beta version too.

I haven't looked into this yet, but it would be helpful to get a full stack trace out of the logs instead of the very basic information in the chat messages,

@Kobata, there ya go: https://gist.github.com/GirafiStudios/02457952e424c2b61c07

Huh, interesting, I think there's been a bug in the anvil slot assignments for a while, it just wasn't easily noticeable before.

That should fix it -- as a note the input slots on the anvil will be emptied by inventory sort like vanilla crafting tables as they are temporary slots.