Brook's law
Adding manpower to a late software project makes it later.
Explanation:
- Ramp up time for new members.
 - Contribution from existing members to onboard new members
 - More time spent on keeping everyone in sync
 
Solutions:
- Change the deadlines
 - Bring specialists who don’t need to ramp up.
 - Good modularization of the problem.
 
References
Book: The Mythical Man Month