CoreDataErrorThe model used to open the store is incompatible with the one used to create the store

1. You created some entities with some attributes and wrote some code
2. Launched the app, probably added some content
3. Quit the app and added/changed some more entities with attributes
4. You probably launched the app again and now it's giving you the error

The reason for this is because your new managed object model is trying to use older version of storage (the one first time created when you launched the app).

The quick and dirty fix would be to remove the storage file (somewhere in ~/Library/Application Support/YOUR_APP/) and to launch your app again.

For future reference - if you release an app and in next release the app has changed managed object model - you have to write migrations for it. All this and more is covered in core data programming cookbook in apple documentation.