Five Pound Bag
Martin Fowler writes about project planning and why the Agile way is the better way. He compares bad planning with trying to put 10 pounds of shit into a 5 pound bag. No matter how hard you push, you can't get the 10 pounds into the bag. I've seen 6 pounds go in, but never 10 pounds.
There are also managers that try to get 20 pounds in the 5 pound bag. Although to me totally illogical, they seem to think that, when you place four developers on the project, they can divide the 20 pounds by four, resulting in 5 pounds. That's exactly what fits in the bag.
As Fowler says, you have to make room for everything, especially new features. When you want to add a new feature, you always have to ask 'what do I take out to make room?' Fowler says that when you see features being added within making room, planning is done badly. This is so logical, at least from my point of view. How come project managers don't follow that simple rule?