Thinking that I had recalled all of my anomaly-exploring sensor ships, I upgraded them into fighters without sensors. However, one was still out there, on Auto-Survey, and on its way to the last anomaly of the map. When it got there, though, it didn't have sensors, but was still set to auto-explore anomalies, and so it made it to an adjacent square, tried to move onto it, popped up the message that only ships with etc. can do this, and then repeated that move after I pressed Done. I couldn't get out to the menu, even, and had to Alt-F4.

I know I made a mistake, but I doubt I'd be the only one, ever.