So I'm currently doing the entire Thieves Guild questline, including the side quests for Vex and Delvin. I did three of the special jobs for Delvin, thus resulting on getting the merchants down in the Flaggon.
Here's the bug.
Every time I go to Delvin looking for extra work, the dialogue comes up that I've completed the three previous special jobs, and he continues to pay me for them. Pretty sweet.