One of my former managers told me this story from a project he worked on many years ago. This project was broken up into multiple groups, and there was a weekly meeting where representatives from each group got together to discuss how the project was going.
One of the groups was responsible for generating the reports and analysis. This was an important part of the project, but not a part that other groups depended on, since the reports were "pure output". At each meeting, the reporting and analysis group indicated steadily improving progress. Ten percent complete. Twenty-five percent. Fifty percent. The number increased week by week. Things looked good.
And then one week they reported that they were eighty percent done, adding, "It almost links now."
I don't know how everybody in the room reacted to this revelation, but I suspect it was met with stunned silence. It's a good thing nobody had a dependency on the reports.