Something to share today. When migrating a content Project 2007 content database, I discovered the SharePoint content database which contains all the project sites would not upgrade if you already have an existing instance in the same destination SharePoint farm.
The scenario was I upgrded the content database in its own web application. Worked perfectly. Then for another test I created a brand new web application, restored another copy of the 2007 content database into SQL. Then ran the stsadm -o addcontentdb command to upgrade the content database. After it completes (with errors, but this is normal) I notice the site collection did not upgrade. Further investigation showed it did not even register according to the SharePoint GUI in Central Administration. This would continue to fail until I actually removed the previous web application
Now this is pure speculation but i suspect the issue is because the SharePoint sites come accross with their existing unique identifiers (GUIDs) and SharePoint may sense these IDs are already in use and therfore discards them in the second instance upgrade. Now, you may think, WOW, that's amazing Piet! but let's face it why on earth would you want to do this in a real world scenario anyway? Well I did and thought it may be handy to know for someone else experimenting with Project Server upgrades.