The lesson: Estimations of work to be done are rarely correct. 
Typically a person will “estimate” that the work involved is actually 30% under the actual effort required.
An example was recently given to me of how a person can expect that a task is thought to be straight forward. The estimator can assume it will be easy also but then both parties find that it’s not. It’s a simple story. It doesn’t involve an IT project, managers, stakeholders etc, but still illustrates the issues we can hit with estimations.
The story involved changing a light bulb. “Just” changing a light bulb! you say? Yes well the stakeholder (wife) asked the project manager/engineer (husband…no not me 🙂 )to “just” change the light bulb. It’s assumed this is a simple task.
So the light bulb was found and ready to go. First task was to unscrew the current one. Twist and turn. Twist and turn. Twist and turn……nothing… not a budge. So he tries a little harder and the bulb smashes. Oh dear! An unforeseen problem, Imagine that!
So he ascertained that a pliers was needed but didn’t have one. So a trip to the local hardware store was required along with the purchase. Back home pliers in hand (power turned off to avoid electrocution) the connection was tackled. In tackling the connection though pins were bent out of shape. So this meant that the new bulb would not fit anymore.
Another trip to the hardware store and a purchase of a new connection was required and the tools to extract that and replace it.
So with the connection fixed, new bulb attached the switch was flicked and ….nothing, nada, zilch happened. Note the use of a grizzly bear image to add visuals to our story…
Anyway, the bulb is taken back out, pins checked, bulb put back in, switch flicked again. Still nothing, head scratching and some coffee later he realizes that he didn’t turn the power back on. Once he does the light comes on… finally!.
So “just” changing a light bulb (a few minutes work) turned into a full day of work/effort.
Worth remembering the next time you are quoting based on an assumption I think.
Note : No one was harmed in the telling of this story, characters are all fictional I swear!!!