All code in the web folder must be supported by SVN commits, right?
If so, you can easily track the problem by simply looking at the SVN logs for the last few changes that have been committed. svn info will tell you which edition the working copy is in, so you know where to start looking
Once you track the commit with an error in it, you can use svn blame to find the person who did it and explain to them what they overlook and how they caused the error. Then you can get them to buy all the dinners for ruining the site.
If you locally modified / added any files that are not in SVN, then svn stat and svn diff will show you what these changes are, so you can find out if they caused a problem. Then you must undo these changes so that your working copy is a clean check or commit changes to the repository.
There is nothing worse than trying to track down an error in your code, only to find out after 3 hours that the error is not actually in any of your codes, but in some stupid local setting someone made in a working copy that never turned out perfect: - (
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