How can I prevent the "You have mixed tabs and spaces. Fix this?" message?

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).

Update for Visual Studio 2017. If you want to disable the "You have mixed tabs and spaces. Fix this?" message, you should go to

Tools → Extensions and Updates…

In windows which popped up, locate Fix Mixed Tabs (image below) and either click on Disable or Uninstall based on what you want.

Related: If you do want Productivity Power Tools to notify you of tabs/spaces, but you keep getting the message right after you clicked Fix this then look in Tools > Options > Text Editor > [language] > Tabs.

Set this to Keep tabs

If you have it set to Insert spaces and you click Keep tabs on the Power Tools notification then VS will re-insert spaces after save, causing the notification to immediately pop up again...aaand now you're stuck in a loop.