First scenario: you broke the promise and you'll have to apologise before your boss. If you do this too often you should prepare to get fired or something.
Second scenario: you don't tell them you think it'll take 3 days, you tell them "it'll be done in 3 days" instead. You tell them it'll take a bit longer, and then you do it again, and then you do it again. A month has passed by now. Damn right your customer is going to be at least a little bit tipped off at that point. Because that's a pretty big estimation error at that point.
Anyway, it doesn't even matter anymore because the new theme is right around the corner, right?
And I'm sorry for getting those two things mixed up. Truth is that the theme has been "around the corner" for a looong while now.