I'm using alt-tab to switch applications on Ubuntu 16.04 and fully updated. There seems to be a screenshot in the way of the usual icon of the apps I'm tabbing through. Then, the application being switched to is the screenshot one not the icon.
It's due to Unity and Compiz's application switchers both being enabled with the same keybinding (alt+tab).
You can disable the Compiz Application Switcher by:
sudo apt install compizconfig-settings-manager
Open the CompizConfig Settings Manager by launching "ccsm"
Select "Window Management"
Uncheck "Application Switcher"
Now, only Unity's application switcher (Desktop > Ubuntu Unity Plugin) should appear when you press