I can't believe I've been playing TES games for close to 10 years and I've never been on here before
So anyway, since I got the platinum on Skyrim I decided to start over so I could play from the beginning with the DLC.
I was doing the Thieves Guild quest line and something pretty cool happened. After I got the Falmer Eye and Skeleton Key from Mercer, Brynjolf and Karliah didn't leave the Waterside Cave. So, now I'm stuck with the Skeleton Key.
I made my way to the Twilight Sepulcher and saved it before I entered. I worked my way through the quest and got to the end. Everything went fine. I go back to the Ratway Cistern to be initiated as the Thieves Guild Master and Brynjolf isn't there. He's STILL in Waterside Cave.
So, I loaded up my game from the save and now I have the Skeleton Key permanently. Thank God for Legendary Mode. This'll make leveling up Lockpick MUCH easier.
It's the first glitch that actually benefitted me lol.