Planning Meeting–What went wrong?

Today we discussed how to develop a new feature that is needed to improve some performance issues. The interesting part took place in the end of that meeting.

The project lead asked: “How long would it take to finish that feature. Would it be possible to get it running until Wednesday?”
The developer answered: “We’ll complete that feature today!”
I argued that a one day estimation would not be applicable for such a task, regarding testing, deployment and so on.
The project lead answered: “I know. When he is saying he needs one day to accomplish a task, he actually will have done it in three.”
Both nodding their heads.

So I tried to explain what my concerns with this conversations were.
I told them that it is not acceptable to don’t let others know what they are doing wrong. If I won’t ever say that his estimations are totally wrong, he’ll never challenge his estimations.
So I tried to convince them that they both handled the wrong way. The one for the overhasty estimation and the other for his ignorance for a mistake.
Frankly speaking, of their reactions I didn’t feel understand, but never mind, this situation will recur and I will try to indicate that again.

It is not about whether the one day estimation nor the three day estimation were wrong or right. It is all about communication.
Don’t commit to something you aren’t sure you can stick with and point out others mistakes (in a gently way).