I'm not sure if this has already been posted, or reported, but I couldn't find any topics on the matter.
I just found an interesting bug with Nephalem Rifts, which allowed me to do a consecutive rift after completing one for no key fragments. I was getting impatient as a Rift was counting down to close, and ended up sitting right on top of it, waiting to click and channel again as it closed up. The second the name of the portal changed from the Rift name, back to 'Nephalem Obelisk' I clicked it.
However, it didn't do that, for a split second, it changed to 'Nephalem Rift' and I clicked on it. Once I did, I was taken into a completely new Rift, filled with monsters again. Orek said his little text, however there was no bar to indicate when the Rift Guardian would spawn, but it did end up spawning after I killed enough monsters.
So everything essentially went as a normal Rift would, when I finally finished, I wasn't given any completed quest gold/experience, and there was no countdown for the Rift to close. What was important, however, was when I opened another Rift after that, it was the same one I just did, completely empty, with no monsters.
So from what I can tell, the game creates another Rift right after the one you've finished is done counting down, and for that split second, you're able to click on the portal again before it turns back into a Nephalem Obelisk, and get into that Rift, without spending any Fragments. From there, once you finish the free Rift, you can probably leave the game, and go back in, to make sure you don't waste any Fragments opening an empty Rift.
Not the biggest bug in the whole world, but if you can time it right, and click on the portal during that split second, you can essentially get two Rifts for the price of one.