There is someone who OWNS the estimates, usually manager. So I just tell him that his estimates were wrong, same way I would open new bug report on software. Fo

There is someone who OWNS the estimates, usually manager. So I just tell him that his estimates were wrong, same way I would open new bug report on software. For my own customers I provide revisited estimate and plan B if they decide to pull the plug. There is lot of stuff about honesty and so on. After being maintenance programmer for a long time, I would just say: Do not stress about it, and always inform relevant people.