So this happened the other day; shouldn't have happened if I had been taking my time and paying attention to what I was doing, but it did:
I was cleaning a bunch of ropes from the top of a short crag, there's trail access to the top of this crag, so after everyone was done climbing I climbed up one last time to clean all the anchors. On the last anchor, I cleaned the gear I was using, and quickly brought the one end of the rope up so I could feed it through the ring on the belay station anchor so I could rap down instead of hiking out. I was rushing a bit because everyone else had already walked off except one person, and truth be told I didn't have much on me at the time (long story short, I was climbing barefoot in a variation of one of these, and didn't bring a descender with me). So I feed the rope through the ring and drop the end back down, then quickly tie a munter and jump off the overhanging top. I got about a halfway down when I noticed that one of the ends of my rope wasn't touching the ground... In fact it was probably about 10-15ft above the ground. After rolling my eyes and chastising myself in my mind for being so careless, I had to decide what to do in order to prevent myself from running out of rope and taking a fall (I was free hanging and a good ways away from the wall).
Obviously, I should have confirmed my rope was on the ground and knotted the ends, but what I should have done couldn't help me after it was too late.
I know this is something that can happen when descending on two different sized ropes that are doubled up, because the smaller diameter rope will go through your device faster that the larger rope, and ergo come up short when you get to the bottom.
Despite whatever circumstance may put you in this situation, what is the proper way to get out of it?