[…] AppFog currently does not offer a persistent file system. As with most PaaS providers right now, all persistent data is expected to go in a database.
What this means is that every time your application restarts, all files revert to the state they were in when last uploaded. Most WordPress activity, such as posts and comments, lives in the database and requires no special consideration. However, if you install some plugins or themes, or any other updates which write to files, then all those changes will be rudely rolled-back. Even worse, the database will not be rolled-back, so your WordPress files and data may be out of sync and broken.