Sometimes websites produce warnings or error messages without giving any clues what caused the issue. Surely this has happened to everyone and it is annoying, especially if it happens at a website you are supposed to maintain.
Recently someone reported that they are unable to update articles because every time they try they get the message "Warning, Registration failed. Please try again." Clearly this message does not belong nowhere near the editing process where articles are updated or created. Something must have gone wrong in a third party extension or a home grown plugin.
If you have a similar situation where a seemingly unrelated error or a warning is reported in the browser, search for the text string from the website or clone or simply download the filesystem of the site and search on your workstation. A utility like grep or Windows grep are the easiest to use.
The text of the message is most likely to be found in one of the language files. This of course assumes that the extension that produced the message follows Joomla best practice of keeping the message texts in separate files, consisting of language constants and language strings.
Third party extensions add their own language files into the languages folder either at the front end or the back end of the site, or both. Extensions can also install their language files in a subfolder in the same folder where the extension itself is installed. Therefore a text search utility is the best approach to find any obscure messages where the origin can be anywhere in the system.
Once you find a match in a language file, copy the language constant from the start of the line and then search for that constant from the website filesystem or its copy exactly the same way, using a utility like grep or Windows grep. This way you can identify the file or all the files where the language constant has been used and you will be able to identify the extention.
If the message is not defined in one of the language files, it would have had to be hard coded in a third party extension, for example a plugin, which you may then be able to unpublish.