Sounds like you have the Visual Studio Productivity Power Tools 2013 installed. Go into its settings and turn off “Fix Mixed Tabs”…
Tools -> Options -> Productivity Power Tools -> Turn Extensions On/Off
If you have the default VS settings, tabs in the editor are converted to spaces. So (in theory) if you clicked Untabify when the message comes up, it should convert all tabs to spaces and the errors should not appear next time. Are you working with other developers who have different tab settings in their IDE?
UPDATE: In Visual Studio 2017 and onward, this is a stand-alone extension named “Fix Mixed Tabs” that can be disabled or uninstalled from “Extensions and Updates” window (Tools > Extensions and Updates).
