When you had a whole session planned, but your players just keep RPing
(comicpress.socksandpuppets.com)
Ice-cream is not anachronistic in your DnD games, it was invented over 2000 years ago in Rome.
My players did this last session. I'm home brewing, and I've given them all bits of the plot individually to this point, so they rented a room and hashed out everything they knew themselves so that they could better understand the world. I got a little choked up. I had combat and everything planned, but I'll take some good RP any day of the week.