The Costs & Risks of Poor Change Management by Luke Garner
Failure to understand the people side of change is akin to failing to learn to break when learning to drive. Sure, you'll probably get to where you intended to go, but you'll be battered, bruised and have a trail of angry people behind you.
All of us, at some point, have experienced a poorly delivered change. Either a technical change or an organisational change, either as the culprit or the victim.