re: "It is a real pain in the ass when you use a square only to discover
it isn't."
I have a Delta 10" miter saw. After a few years of rough use, I wanted
to make some cuts that had to be square. When I tried to square the
fence to the blade, I found that I could only square it on one side of
the blade or the other - not both.
I took the fence off and laid it face down on my table saw. One side
was perfectly flat on the table across it's whole face. The other side
touched at the far end but not near the middle. It was up enough that
I could slip a business card under the face with no resistance.
I called a local machine shop and luckily ended up speaking to a guy
that was a weekend woodworker like myself. He completely understood the
frustration of not being able to square up a tool and told me to bring
the fence in. For $20 (which probably went for coffee and donuts) he had
one of his guys mill the face completely flat. It squared up perfectly
after that.