If there's only one answer, then this must not be a very interesting topic.
It seems to me to be important to distinguish a good idea from poor implementations of it
The lesson is: Even if you know exactly what is going on in you system, measure performance, don't speculate. You'll learn something, and nine times out of ten, it won't be that you were right!!
Code never lies, comments sometimes do.
The wages of sin is debugging.
One of the great skills in using any language is knowing what not to use, what not to say. There's that simplicity thing again.