I am running Ubuntu 13.04 on my 13" Macbook (2008 model) and I have been running it on 13.04 since its release and have not had one problem with it yet. About 2 weeks ago I began messing around with Unity Tweak and so no issues. Today, when I go to minimize an application (Happens to any application, regardless of minimized or windowed), it freezes halfway during its minimization. The application's window is translucent and the computer will not respond UNLESS I press the eject button on my keyboard (I randomly pressed it to see if it would respond to it). Once I press the eject button, the window minimizes and I get control again. I have removed Unity Tweak and I am still getting issues. It doesn't necessarily hinder the system, its just REALLY annoying.
I had the exact same problem, I have an HP DV4 laptop i5 4Gb and I recently installed Ubuntu and did all the updates and upgrades necessary to get to the 13.04 version I wanted to experience more and installed Unity Tweak Tool, I played with it a bit and when I thought I had set everything according to my preferences, Pain came to my brain!!! everytime I wanted to minimize a window the computer started to run slow and the fans went to max speed, I could only close the windows by pressing constantly ALT+F4, I even forced laptop to turn off by pressing the Power Button and when the laptop restated The login box never disappeared and the wallpaper wouldn't change either, right click on the desk and the contextual menu would't show, Anyway...
I opened the Unity Tweak Tool again and started to try the controls one by one and BINGO!!! the problem is in the SYSTEM section, DESKTOP ICONS and ICONS ON DESKTOP.
For some reason if you switch off the icons, everything goes down, so my suggestion is, reinstall the Unity Tweak Tool and go for the ICONS ON DESKTOP option, everything should go back to normal right after you do that.