I had two SQL Server Instances connected in my Object Explorer (A and B, both with identical database names and schema but different data - test and production).
When I ran a query in the query window on my A instance, then disconnected from A and ran the query again it continued to pull results from A despite having already disconnected.
This created a lot of confusion because it appeared that I was connected to the intended instance B but was returning results from a previously closed connection. A simple solution was to close the old query window and open a new one but this wasn't very obvious and took a while to figure out.
A nice fix would be to display a message to the user saying that the connection to that server instance was already closed or have it automatically switch to the only remaining connection if the names are the same.