Symptom: Estimates of the basic work are realistic, but people forget to account for related deliverables that are required to produce a quality product.
Clients judge delivery not just based on the basic functionality required, but based on the entire suite of features that comprise a quality product. If estimates don't include other deliverables like project documentation and installation, the end result won't be delivered on time.
The problem may be a lack of a clear definition of what is meant by quality.
Another (more generic) way to state the problem is as follows:
