Premature Ramp-up, Martin Fowler on the perils of building up a development team too quickly: loss of code cohesion, breakdown of communication, plus the business costs of on-boarding. The problem I’m more concerned with, when growing a software team, is maintaining culture.
Adding a new person to a team is a process of integrating the new person’s unique good qualities to the team’s existing culture. It’s critical to use their prior experiences to clean up the sharp edges of the existing team practice without accidentally integrating new sharp edges. It’s a careful balancing act of taking advantage of the beginner’s mind and cultural indoctrination. Both sides have to give and take.
If you grow too quickly, it’s very easy for this balancing act to get, well, out of balance. The new people are only indoctrinated and the team doesn’t learn, or the new people don’t understand the team and go about doing whatever they felt was successful at their previous gig.
Its common to focus on the difficulty of recruiting a team, but finding a culture match and growing that culture is equally, if not more, challenging.
Adam Keys is typing 
I especially like the point of having a solid core of code before ramping up because people will likely copy what’s already there. I see that constantly at work, with new hires doing something in a way they know is wrong because they see it done that way in existing code (most of which we want to refactor “when we have some time”).