The most common error that occurs is: OraForms: 5 - Handler not found.
This error is thrown if the control (handler) that is referenced as the first parameter of a failing OraForms API call cannot be found, for example a button that should be pressed is not on the form.
By having a look at the preceding calls - prior to the one that throws the error - it is often easy to see why the control is not there. Oracle Forms applications often send a WINSYS_BEEP message if an error occurs, for example when entering invalid data. Some applications also update the status bar message. Moving through the preceding calls and looking at the Info tab will show you if there was a WINSYS_BEEP or a status bar update.
Very often, the server updates the status bar message in case of an error. Check the preceding calls, prior to the one that throws the error, if there is any information about a changed status bar message. Whenever the status bar message changes, an informational log entry will be logged.