I am runnung Ubuntu 12.04 LTS. If I have a few windows open (say four to six, not a large amount), and minimise all by pressing Ctrl+Super+D, they all minimise immediately with no problems. However, if I minimise them one at a time, by clicking the minimise button with my mouse, the last one often hangs.

Usually it will appear as a ghost on the screen for a while, semi-minimised (in other words, shrunk toward the Unity launcher bar, half-size or smaller, and semi-transparent). Usually it will eventually clear; sometimes the computer just freezes and I have to restart it.

It doesn’t seem to matter what the window actually is. Just now, I had to restart my computer with Skype* semi-minimised. I’ve seen it freeze before with the Terminal semi-minimised (and the Terminal wasn’t even doing anything at the time). The only pattern is that it’s always the last window minimised which freezes, and that minimising all windows together using the keyboard shortcut works fine.

What on earth is going on, and how can I stop it?

_{* I hate Skype, but I need it for work.}