As part of removeShard we perform a series of checks such that there is no user data left in the database that we just moved.

However, these checks do not take into account that some collections inside the database are actually unreplicated and thus node-local. This is the case of the <DB>.system.profile collection.

As a result, having that collection present on the primary shard for a database and attempting to remove the shard will result in the process hanging indefinitely.