Is this an error that the average user will see while using the site normally, or is it a silent error that you will only see if you check your debugger? If it's in the latter category, it's only a bug in a fairly academic sense. If silent, meaningless JS errors are what it takes to make the site work across browsers, I would not consider that a failure of testing. To the contrary, it's efficient allocation of testing resources.
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Is this an error that the average user will see while using the site normally, or is it a silent error that you will only see if you check your debugger? If it's in the latter category, it's only a bug in a fairly academic sense. If silent, meaningless JS errors are what it takes to make the site work across browsers, I would not consider that a failure of testing. To the contrary, it's efficient allocation of testing resources.