Answer by Josh for Visual Studio 2010 annoyingly opens documents in wrong MDI pane

Did you perhaps use the "New Horizontal Tab Group" command to split #1 into #1 and #2, then move your errors window into that?

I would just try resetting the window layout and see if that fixes it.

1. First, Window -> Close All Documents
2. Then, Window -> Reset Window Layout
3. Exit VS to be sure, then go back in.