What is the most neglected phase in the project lifecycle? Why?

Closing is the most neglected phase in the project lifecycle. Because when the project is done, everyone on the project sighs relief, especially the project manager.

Project completion, documentation and lessons learned - almost no one ever does that.

Out of my experience there are two phases that are more or less neglected.

Project Plan: Design phase, there where the Project gets his costs, building time and construction principle first everybody is thrilled by the new Project but forget the importance of the Phase.

Also with the Procurement Plan: the Project is (or about to be) delivered and everything is more important

Project completion.

Once the project is over, it’s easy to pack things up, throw some files in a drawer, and start moving right into the initiation phase of the next project.

I would say completion stage is the most neglected phase in the project life cycle particularly if there had been problems in the course of construction. At the completion stage, everyone is anxious to end the project and move to new project.

At the completion stage, hardly any care for the finishing work in a hurry to shift to the next project.

Mainly because cost control (but the importance of the finishing work are often neglected) and cost reduction--but later on it may require more spending to recoup the important details as a part of the "evidence requirements" in case of litigation s often associated with certain procedures.

The project closing phase is the most neglected phase in the project life cycle.

By the time the project closes, everybody just relaxes.

I'd say by far the project closing phase is the most neglected phase in the project lifecycle.Why?Because when the project is done, everyone on the project sighs relief, especially the project manager. But when the project is done with the execution phase, it doesn't mean that it's finished and should be closed. The closing (or closure) phase should not be taken lightly by the project manager, as it has, besides the logistics of administrative and contractual closures, on the most important activities in a project: "The lessons learned".

In reality, a poorly planned and/or poorly executed Implementation could make or break a new system. It's a pity to see all kinds of time, attention and money go into the development of a new system only to see it "crash and burn" because the Implementation was haphazard and shoddy. 

I agree with above answer, it's closing phase, because every body just focus on the next project.

therefore project manager should be careful, in order to keep project on track he need a mix of discipline and leadership to manage his team towards the stage.