My buddy down in Tucson ran into this last spring on a driveway pour. The homeowner came out with his own level and started checking the slope before the concrete truck even showed up. My friend had to tear out two forms and redo them because the guy was right about the grade being off. He said the worst part was the guy stood there the whole time watching him fix it. Now he keeps a cheap torpedo level in his truck just so the customer can check it with his own eyes and feel good about it. Saves him a lot of grief when the guy thinks he found something first.

See, I get what you're saying, but three degrees off is still pretty small in the grand scheme of things. I've been shooting with a polar alignment that's probably off by that much for years and my normal shots come out fine. You only really notice it when you're doing super long exposures or tight crops. I mean, sure, do the drift test if you're bored, but saying it was "the real problem all along" feels like a stretch. A lot of times people just need to shorten their shutter speed or stop pixel peeping at 400%.