A common use case for sandbox is to seed the database with an OSM extract. In doing so, the import script will create users in the database for any usernames found in the extract data. This is fine unless someone with one of these usernames tries to login to the sandbox, in which case the signup will fail and the user won't be able to map in the sandbox. This will be much more common than random chance, since many sandbox creators will likely be working with OSM areas where they have done some mapping.
A common use case for sandbox is to seed the database with an OSM extract. In doing so, the import script will create users in the database for any usernames found in the extract data. This is fine unless someone with one of these usernames tries to login to the sandbox, in which case the signup will fail and the user won't be able to map in the sandbox. This will be much more common than random chance, since many sandbox creators will likely be working with OSM areas where they have done some mapping.