Normally, when the dialogue calls for "Goodbye" or any other forced response from the player, the game pauses until the response is given. Clicking on any other topic, or pressing any key, or moving the mouse has no effect.

However, there was a mod I played some time ago where this pause somehow got overridden. If the player got near one particular NPC, a dialogue window would open and the greeting "Who's there?" would appear. Below that was "Goodbye" in red. At this point you would expect the game to pause until the player clicked on Goodbye. But instead, the "Who's there?" and "Goodbye" lines just kept following each other in a steady stream, with no pause between repetitions.

Obviously there was a bug in the mod, and I ended up deleting it. But what I'd like to know is, how was it possible for it to have broken out of the normal Goodbye pause and generated another dialogue line? It might be useful to be able to do this on purpose.

It would most likely be a ForceGreeting bug. If it's called every frame without a menumode or doonce condition then it just spams the greeting over and over and can't be stopped.