Suppose we have 2 shards: shard0 and shard1 and 2 routers: router0 and router1.

From router0 we create an unsharded collection making sure that the primary shard is shard0. If we issue a write operation from router1 to that same collection, it might be targeted to shard1, which will cause the insert operation to fail. Refer to the build failure comments for more information.