In terms of the RBDM moments of which I'm most proud, there are a couple that stand out that took place in rapid succession in one particular campaign. These are well documented in my Faded Glory Story Hour if you care to read it.
The PC's had been plotting for quite some time to get to and inside this mysterious city in a large crater from which armies of Bane worshiping Orcs had emerged. They knew that the place was also guarded by Ogres and at least one Beholder in addition to some Banelar (basically snake wizards if you're not familiar with them).
They had meticulously planned this mission with whatever information they could get but they had been unable to scry upon anything inside the crater for some strange reason. Nonetheless, they knew that they would meet tough resistance when they got there and would need to make a speedy exit at some point. Thus the party Wizard had prepared a Teleport and they even had a "code word" for when he would get them out of there: Albatross.
So they get inside and they are battling Ogres in plate mail and doing pretty well and finally grind their way forward into this large chamber when suddenly a hole is disintegrated in the roof and in flys the Beholder. The Wizard yells "Albatross!" and the party converges on him. A moment later, just as the Beholder is about to bring his full attacks to bear upon them, they teleport out.
Into Hell.
The reason they couldn't scry upon the city was that it was the site where a god had been "killed" (Bane) and that caused a "weak spot" in reality right there that was linked to where Bane's Tomb in Hell was constructed. The PC's got a good look at the horde of demons who were gradually tearing apart this tomb and knew that that couldn't be good. The party immediately implored the Wizard, "Send us back!" He said, "I can't! I only prepared ONE Teleport spell today!" They needed a comfy place to spend the night. In Hell.
(This brings us to the second RBDM moment...)
They found a very helpful Erinyes who was happy to let them share her cave for a very small price: A kiss. After I described her as the "unholy love child of Monica Bellucci and Salma Hayek" the Rogue immediately said, "I'll do it!" The Cleric of St. Cuthbert said something akin to "I've got a bad feeling about this." as he layered on whatever magical protections he could think of to keep the Rogue from selling his soul to her.
And it worked! He rolled really well and all she got was a kiss. She invited them into her lair where she settled them down in the antechamber and then seductively told them, "If the floor isn't comfortable...my bed is right through this doorway." The Rogue looked around the group and basically said, "I'm going to go and...uh...pump her for information."
The Cleric objected to what an absolutely terrible idea that was whereupon the Rogue gave the group his best Han Solo impression saying, "Relax! It's me!" Then he merrily joined the seductress and sold her his soul. For cheap.
He was later able to pay off his debt to her in small installments at horrible interest rates. But he did it with a smile on his face.