I'll list in order what I need to do to create the bug:
1. Enter a public game, selecting any available quest from Hell difficulty. Since I am level 60, this means the last quest of act 2 through the last quest of act 4. Let's use "The Prime Evil" as an example.
2. Leave the public game.
3. Open the quest select menu for public games.
4. "The Prime Evil" is now a selectable option on Inferno difficulty and I can join that game.
I'm not sure what happened to make this bug appear in the first place - when I logged on earlier this evening the game was working normally.
Edit: can confirm that this bug also enables me to create a single player game that skips quests/acts.