They say that in theory, there's no difference between theory and practice... but in practice there is. I wish, truly and deeply, that they would stop. Just, stop saying it.
The worst part is how much wisdom a saying like that might conceal – if it only failed to enlighten the listener it wouldn't be that bad. There is a lot of systems-imagination going on, people attempting to describe things before they're even familiar with it. Or maybe they don't know how it feels to synthesize an understanding, they don't understand it's supposed to feel bad and weird to leave out all the details.
There's a lot of room for improvement, lots of wisdom to share... all of which gets overshadowed by a common saying that's just admonishing people for being arrogant.
I'm sure that the problem exists, of people not checking how they imagine the world against the world itself. But the saying is never phrased that way; they aren't warning people against, "if a theory says there's no difference between theory and practice...". It admonishes people for having a concrete descriptive understanding of the world... but why? It would be like criticizing someone for trying to articulate their thoughts in words – how else would you share an understanding? Obviously it's imperfect, but isn't the whole point that abandoning imperfect things is ridiculous?