There is an issue where if you sold the Thieves Guild Armor to Tonilia when you get to the point in the Thieves Guild quests where she asks you to exchange the armor for an upgraded version there is seemingly no way to get the armor back. You lose the option to deal with her so you cannot buy the armor back, nor can you steal it or take it off the bodies of any other Thieves Guild members. This makes it so you are unable to complete the quest and gain the achievement for returning the Thieves Guild to its former glory. You also lose the ability to sell stolen goods to Tonilia.

Would improving the thieves guild armor also cause this problem?