The Mythical man-month

The Mythical man-month is the idea that “Adding manpower to a late software project makes it later.”. Unexperienced managers and others who are desperate to reach time-lines always try to throw more people on a project to speed up development. However, because that new person has to be trained, and ask questions about the project, he ends up slowing down the entire project even more. Fred Brooks wrote “The Mythical man-month” book in 1975, he recognized this while managing a project at IBM.

Wikipedia has this covered pretty in depth, so read more about it here.
http://en.wikipedia.org/wiki/The_Mythical_Man-Month

I have experienced this several times first hand - and this situation is hard to avoid, however here are a few pointers that will help you avoid this situation.

• Recognize when you start getting behind. Waiting untill the last month (or week) of a projects deadline to ask for additional manpower is the defination of this problem. The sooner you add manpower to a project, the more productive days the bigger team will have. Say you have a project with a 6 month deadline. If after 5 months you realize your behind - it will take a good 2 or 3 weeks to get additional people up to speed - and you will only have 1 or 2 productive weeks left. If you were to add more people after say 3 months, you would have 2 1/2 productive months with a bigger team. Basically, the soon you add more people the better.
• Overstock your team. Take additional time to plan how many people should work on a project. If you’re unsure - it is better to have to many people working on a project (to an extent) than to few. You can easily take people off a project if your ahead of schedule without blowing the time line. Plus, if the project then gets behind - that same person is already up to speed and can re-join the team with minimal setback.