We have a custom field which is mandatory on a custom sales order form. Despite this, one of our orders was approved and fulfilled without it being checked.

The field is made mandatory by a workflow because it comes through web services and we can't have it mandatory on the form itself or they will fail to come in, as we don't know what the mapping of that field will be.

Any reason why it would fail? It's set to be mandatory on Before Record Load and typically will go through several edits before it's finally approved.