Hi Jason,
I hope you are talking about ExecuteIfSerializedAndNotZeroCount execution rule. I have not modified this rule. I am using the same rule which is there in standard SAP IM 3.2 application.
Below are the screenshots for the SerialNumberDeleteIfNull sub-action:
I am navigating to MaterialCount screen by clicking on "Count" button in Physical Inventory Document line item screen. Here Material Count screen has two buttons "finish" and "done". When I click on finish button I am not getting any error and I am able to see the two messages ( Message8 & Message 9) which are there in SerialNumberDelete action. When I click on Done button (after completing finish button action) I am getting error "Target not found for Transaction SerialNumberDelete". In this case I can see only Message8.
I am using standard SerialNumberDeleteIfNull sub-action.
--Shyam