IME it's usually a change in gradient, or a false equivalence close to the road, that throws me off.
Climb a few miles at a steady 6% grade, for instance, and where it flattens to 2% looks either flat or even downhill. My daughter embarrassed me near the top of Washington Pass (in Washington, of course!) when I mentioned it was getting easier because I could coast the rest of the way. To prove it, I stopped pedaling. And stopped rolling. Whoops, not there yet!
And a road near here was an instance of the latter. Where the road's grade dropped from 5-6% to 2%, the cut at the side of the road angled up steeply. You had to ride a bike (or drive a stick shift) to prove to yourself the road was still going up, your brain made you think you were headed down.