In the _Xref database built during data collection, controls & control source are stored.
When in the course of studying a legacy database, one finds an odd construct in a control source,
the control source of all controls can be queried to locate additional
occurrences of the odd construct.
Notice that some of the immediate if's in the illustration are missing the 3rd argument.
The entire list of iif usage was provided to the database owner/developer.
After all iif's were corrected, the database stopped crashing.