Errors alert messages are logged by pgn4web, such as failure to load PGN data, incorrect PGN games or incorrect FEN strings.
When an error is encountered, a chessboard square will flash to signal the exception.
The error alert log can be reviewed clicking on the flashing chessboard square. When the error log is reviewed by the viewer, the flashing signal stops (until a new error is encountered).
The debugging feature can be used both while developing new HTML pages and when entering PGN data using any of the pgn4web tools, for instance when adding a chessboard to a wordpress based site using the embed-chessboard plugin.
When the game autoplay reaches the illegal move, then the top left chessboard square is flashing and the moves autoplay stops.
At this point, clicking on the top left chessboard square shows the error details in the alert log.
More troubleshooting hints are available in the user notes.