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.

It's nice. I've been given the gold from the three jobs too.