So, one of the common topics about broken quests is in regards to being unable to advance the Thieves Guild questline after a certain point if you sold the original armor Tonilia (since you no longer have a piece to trade in for upgrade).

Well, I started playing a thief again for the first time in months, and I discovered that in the training room in the Cistern, there are 2 pieces of Thieves Guild armor just sitting on a shelf. I found those before getting my initial set of armor from Tonilia, and the pieces she gave me stacked with the two I picked up (hood and gloves, I think), so presumably if you've sold yours you can pick these up to trade in to progress the questline.

I didn't see this listed as a fix in the latest patch, so I'm not sure if it was always there or not. Either way, hope this helps someone.

I cant say for sure but I have heard mention that this was added in the last patch, but dont quote me on that.

I'm going to quote you anyway because you are correct. The "backup" armors were added in a patch.

fixed it for you

Oh, come on guys. Be nice.