PowerPivot : When versions get mixed up.

I worked a case today where they were trying to get SQL 2012 Reporting Services installed in a SharePoint environment that also had SQL 2008 R2 PowerPivot installed. This, by itself, is fine and wasn't really causing the problem. When they tried to open the Excel Workbook from SharePoint that had the PowerPivot data in it, we saw the following error: