You can consistently generate this JS error if you attempt to examine the template files on the comment preview page - after you add a comment and submit it for preview. The theme information overlay will not show any information now matter where you click. Each time you click to examine the theme information, you will get another JS error.

I replicated the same error on a clean D6.20 install with only the devel and devel_themer module installed.

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ricovandevin’s picture

ricovandevin commented
Title: JS error: vars is undefined -> on preview comment page » Javascript error 'vars is undefined' on preview page

I can confirm (one year after this topic was created) that this bug is also present in the 7.x-dev version.