While researching quotes I found a book worth scanning on Google Books: 1001 Smartest Things Ever Said, edited by Steven D. Price.
If you’re not aware of Google Books, you should be.
Take a look. It’s free to view and you can download many as eBooks, for a small fee.
A personal favorite quote from the book, and the very reason it caught my attention:
The most difficult thing in the world is to know how to do a thing and to watch someone else do it wrong, without comment.- T. H. White