Somehow ended up in a state where the end turn button was no longer clickable.  I can't think of anything I did to set this off.  I had a pioneer selected and was waiting for him to arrive at a horse tile (the one east of the starting location).  I believe this may have happened the same turn he spotted a neutral group (Hornswaggle maybe).  If it matters he was taking a long route from the south as my city was blocking him.  Pretty sure he was in the forest 2 squares south(ish) from the horse.  Another thing of note was that I Alt-Tabbed out of the game to write this report.  When I Alt-Tabbed back the screen was black but the music was going.  I have Alt-Tabbed out successful before this with this build.  Had to cancel the task to continue.  Game was pretty far along when this happened (probably approaching turn 100).