Unix-Jedi put the last part of this into a comment, and I liked it so much I went searching for the source. The original is, apparently, old enough that the source is unknown, but it’s still accurate today:
Reality is the murder of a beautiful theory by a gang of ugly facts.
Theory and reality are only theoretically related.
In theory there is no difference between theory and practice.
In practice there is.
I may make this the new official motto of TSM!