Reading breaks. Every course is different. I've seen greens that don't bend the ball as much one would think; overdoing the breaks on those greens might leave one well past, but above, the hole. Such greens are typically shaggy and probably not well maintained. However, fast greens will be very obvious, as a slight nudge can start the ball zinging on a downhill lie.
More often than not, it's better err on the side of caution and play more break than one might think. Among other instructors, Hank Haney is a big advocate of playing more break than one believes is needed. Another way to think of it? Pros don't go low!! They miss above the hole on the high side.
Once a putt falls below the true break line, it has no chance of going in. So err on the side of overestimating the break. That way, even if you don't get the speed perfect, the ball still has a chance of finding the cup. Imagine if you had one putt to win $1 million: To give it every chance to go in, you would definitely keep the ball on the high side.