Youtube is the main one that comes to mind, but I've had it happen on some other sites as well that I don't remember off the top of my head.
Also worth noting: as shown in the screenshot I'm still getting issue #69. And the favicons do display when the tab is unloaded, but only if "highlight unloaded tabs" is set to "gray out."
Tab Tree 1.4.8, Firefox 50.1.0 on Linux
Youtube is the main one that comes to mind, but I've had it happen on some other sites as well that I don't remember off the top of my head. Also worth noting: as shown in the screenshot I'm still getting issue #69. And the favicons do display when the tab is unloaded, but only if "highlight unloaded tabs" is set to "gray out."