Well, it's hard to be specific without knowing where or when the error
appears, but I'd guess that it's likely appearing in a browser at specific
web pages.
There would be two reasons for the error message; the first, the cause,
obviously being that the javascript on the page actually does have syntax
errors. The second, which is the symptom, would be that your browser has
script error notifications turned on.
Other browsers mught be more forgiving with the error detection, but the
page here has a screenshot of how to turn off the script error reporting: