People hiring consultants to build them software are generally non-technical. On most projects the following problems may occur:
Problems With Clients:
1) They don't konw what they really want.
2) They are bad at communicating what they really want.
3) Not enough people from the organization are brought in to weigh in on their needs / actual duties.
4) Too many people are brought in, many with conflicting interests. (bike shedding). [1]
5) Critical omissions are made regarding idiosyncrasies of their infrastructure that are never brought to light until far too late in the project.
6) people trying to change the direction of a project late into development
7) complicated/redundant policies or procedures which are incompatible or inefficient to implement using the tools the client requires you to implement them in.
Problems with Consultants / Contractors:
1) Over confident estimates
2) They are bad at estimations / unfamiliar with the tools required to do the job
3) They are bad at communication / rooting out the the real problem early
4) Unable to detect confusion/bullshitting from a client or are unwilling to investigate it.
5) PMs who make promises they are unable to keep
6) PMs who do not understand the details and make decisions without consulting those who do
7) Developers who avoid asking the client for clarification
8) spineless developers/PMs who screw themselves to avoid confrontation
9) Procrastination
[1] http://en.wikipedia.org/wiki/Parkinson's_law_of_triviality
