Recently, that I completed a project that I had initially estimated to around 2 months, in nearly 4 hours. This morning I fixed the few remaining bugs. I looked at the clock, surprised it was still so early and I still had so many hours left in the day.
Now I have more time to polish the details and go beyond the initial goal (I think this scares my manager a bit), but I could (and I believe some people do this often) stop now and all the management would be satisfied.
What's interesting is that everybody bought the 2 months estimate without questions (I almost even believed it myself). This reminded me of my productivity zero post.