Pretty often web applications produce some stubborn issue that cannot be identified easily. Often different modules/plugins are using same resources and this can lead to strange behavior of the system. In some of these cases it is likely for the ill functionality to be functional and not leave any apparent signs for the error. In those cases the problem solver should vision a detailed image of the system architecture and follow the normal flow of the app's functionality just before the unexpected scenario occurs. Additional traces can be put around the last working points so data to be checked for consistency at this point.