Before yesterday, when I was upgrading from Tiger to Leopard I had a more or less unpleasant incident ... Once the installation was completed successfully and rebooted the system, a nice blue screen gave me the 'welcome'.
Fortunately I had the most important files to another hard disk, if not ...
I thought it would be just my thing and should be simply to bad luck, but apparently there are many more people with this problem. The cause seems clear, and the blue screen on Leopard because Tiger had something 'retouched'.
For my part all I had was some icons of the modified system and Leopard was 'cock a mess' upgrading.
If you have applications such as Application Enhaced (users would blame mostly) save your data before upgrading.