I'm not sure if this has already been addressed or is a unique case, but I have an issue with thumbnailing for minimized windows. My possible bug should be rather easy to replicate though:
If I minimize a Firefox window, for example, by going to the top-right and hitting the minimize button, the thumbnail appears as expected. However, I'm on a laptop a lot of the time and I like to use shortcut keys for doing this because it's faster. If I use Winkey + M or Winkey + D, it acts like it has a thumbnail (the program icon gets small and biases itself down and right) but no thumbnail is generated.

Any ideas? It's definitely doesn't bother me at all, but I figured it was still worth mentioning.