This will be easiest to duplicate on a really large map like insane with LOOOOOTS of other races so you have time, but it's existed since beta I think

1. start a single player insane map with a crapton of races (I use like 20+ these days)
2. order of this step seems irrelevant: meet one or more races & research universal translator 
3. click end turn
4. during the phase where the ai is doing it's thing, go into diplomacy screen -and- speak to someone
5. before all the ai finishes doing its thing, cancel back to the map
6. screen will go black with the custom gc3 cursor as expected when the turn finishes
7. sigh & end task because you forgot to wait... again
8. there was one time I got a message about waiting for other player (with an ai in single player game) & just left it sitting while I went to make coffee.  it did not happen after I got back that time