Why does my viewport have a red and white stripe around it?

A red and white striped border around a viewport indicates that the viewport needs updating. 

Changes that affect the appearance of a wireframe viewport are automatically updated. However, changes that affect the appearance of a rendered viewport require the viewport to be rendered again. For example, layers and classes may have changed, or additional objects might have been added to the design layer.

To update a selected viewport, click Update in the Object Info palette, or select View > Update Selected Viewports. You can also update all viewports in the file at once by selecting View > Update All Viewports.