GatewayVm::checkBack checks whether stack will underflow before a rewind look-up. When checkBack will cause underflow, a custom error GatewayVm::checkBack will propagate. 
The problem is underflow panic will happen before revert custom error. This means custom error will never propagate and the function will always panic first.
https://github.com/code-423n4/2024-12-unruggable/blob/9e0e908a207f088dd843fcdc3e4de5fa0edd6c5c/contracts/GatewayVM.sol#L150
Change to int256 index and error InvalidStackIndex(int256 index).
