Repro:
* do a screen grab, or region grab from Mac OSX. (See org.png)
* Open the PNG in Paint.net
* optionally, edit it
* save it as PNG (you'll note the preview is messed up - file size jumps around but mostly says 'computing')
Problem: The PNG saved cannot be reopened by Paint.net (see bad.png) with "There was an error opening the file." and seems to be generally invalid (explorer preview doesn't work, IE doesn't open it, etc...)
Environment: Running Mac OSX 10.6.4 with Parallels 5.0.9370 with Windows 7 x64 and Paint.net v3.5.5
