Problem occurs at least on gtx 960
How to reproduce this behavior (by Caetano Veyssières)
tested on 2.67 and 2.78c
• Create 2 image empties
• Move the second you created above the first one
• Unselect everything
• Press Z multiple times (it switches them)
The sphere that was created before is always above and the one that was created after is always behind (assuming nothing is selected). "