this happens to my mba 13" 2012 i5 once in a while, after waking from sleep, when connected to external Dell U2312HM Display. I'd say once every 3-4 weeks. After a few moments, it goes away.
I remember reading something similar happening on mac minis, and the issue was with the HDMI out. This being thunderbolt-out, not sure if it's related.
Anyone with more info. care to share?