In a typical POST-redirect cycle, user submits a form, data gets saved to the database, and user is taken to a page that contains the HTML representation of the new record. This is how your GAE application might work (and usually does), but with a twist. Instead of the page, user sees a 404 error. Entity is not there. Confused by this, user hits refresh and behold: the page is now there. This is a typical outcome of eventual consistency of Google AppEngine's high replication datastore.
Starting with the high replication datastore, and particularly with its eventual consistency model, can be a bit hard to wrap your brain around until you've actually run into problems in real-life usage. I had read many tutorials and watched countless hours of talks trying to understand it before I modeled my first database schemas, and it still caught me off-guard. Therefore, I decided to document some of the issues I've encountered, and solutions to those issues.