This is why everytime you putt a ball, it doesn't go where you want it to go. Well, that and the fact that if you play golf, you probably suck at putting. That seems to be a trend in golfers. So far, all the complications talked about have only dealt with putting on a flat surface. Now you have to remember that there is no such thing a flat putting green.
Gravity makes a ball want to roll down hill.
This means that you have to adjust for gravity's pull by applying a force in the opposite direction of gravity's pull. If you are putting straight uphill, you will have to apply a larger force on the ball to make up for the force cancelled out by gravity. If you are putting downhill, you will apply a smaller force because gravity aids the motion of the ball. Next time you stand over a three foot putt worth six skins, remember all the physics involved, and then you won't feel so bad when you inevitably miss the putt.