Formula UDF - using a field with the same name as a system field
If you have a UDF with the same name as a system field, then try to use it in a Formula field, the UDF won't be used if it is in the root folder - it will always use the system field.
I suggest either of 2 possible changes:
Firstly, change the way fields are referenced in formulas to use a unique identifier. That way the correct field always gets used.
Or secondly, adding a duplicate check when creating UDFs to at least make the user aware that creating a matching field can cause issues.