Using Xcode 4, I made a little project, nothing fancy just some basic UI stuff. I created it on a Lion machine. I had the folder on Dropbox, and the next day I built the project on an Snow Leopard machine. Seemed to run fine. It complained about 3 compiler settings but I changed those and it went okay. Now I am back on the Lion machine and the app won't even start, it just beachballs. I tried changing the settings back and it still doesn't work. Any clues before I just cut+paste the files into a new project and start all over? It crashes before loading, or loads then crashes when I click on a button. Xcode gives a logic error and then that's the end of that. The build settings were 'Deployment Target', 'Base SDK', and one other one. I forgot what it was.