From OpenSCADAWiki
Placing or editing the widget program does not directly compile it, that is, there will be no error messages in the program if they have a place to be. This is due to the fact, that the direct execution of the program, and thus its compilation, is carried out in the context of execution and at the moment of starting up of the visualisation project for execution. In this case all errors, that occurred during the compilation, are displayed as OpenSCADA messages and the error widgets are not processed. For details on how to debug user procedures see the "How to debug the OpenSCADA project" document.