UUID mismatch detected with the loaded library

Uninstalling and reinstalling both the iOS on the device and the SDK did not fix this for me. The only way I was able to get around this issue was by deleting the DeviceSupport files for the 4.2.1 iOS version, which can be found at:

/Developer/Platforms/iPhoneOS.platform/DeviceSupport/

After deleting the files and restarting Xcode, I plugged in my device and was prompted to restore the symbol files from the device itself - it took about 5 minutes and after this, everything was back to working perfectly.

As Eric mentioned, this is due to the libraries on the phone being different to those in XCode.

I came across this issue when I try to debug an app from Xcode using: iPhone: iOS 4.2 beta 2 XCode: iOS SDK 4.1

However, if I try to debug using XCode with iOS SDK 4.2 beta 2, then it works fine.

From this, I conclude that the SDK in XCode must match the version of iOS on the phone for debugging to work.

Ensure the two match and you should be able to debug your app.