No clue. Both of those windows were on the same machine.
The only thing I can figure is this. Maybe I opened a command prompt via the "Open Command Window Here" powertoy for Windows XP. In this command prompt, I would have typed in 'command', in an attempt to test something involving ANSI control codes per this article
Maybe the "legacy mode" I was one somehow found it's way back to the window that initiated it (assuming it was the same window).
I didn't experience any further problems after closing that window, either.