It’s a best practice to have all custom code write to the event log. Every code execution should produce one and only one log entry. Exceptions (code speak for err) should be written here. For troubleshooting and support you can’t ask for a better tool. The framework exists, EventCombMT helps aggregate, and it allows you to run dozens … or hundreds … of apps while keeping a sharp eye on quality control.
If you see this error message in WSS chances are you have an active reference to custom field type but have not installed the field type. The “/_layouts/mngfield.aspx” page will throw this error in the above scenario. Two possible resolutions: