That occurs when you are surfing from a machine that has Visual Studio installed on it. It may occur when you have other development environments installed, if they are capable of debugging/developing the languages used to create the web pages you’re loading. The error I see most often is “unterminated string”. I’ve never found a solution for it, so I just live with it or surf from a non-development machine.
The bad news is it’s mildly annoying. The good news is it’s a good way to evaluate the competence of the website builders before you decide to do any business with them. Personally, I don’t trust my credit card info to any site where errors like that occur. If they don’t bother to fix simple things like that, they’re probably ALWAYS that sloppy in their coding, and I don’t trust their security measures. But, that’s just my opinion.