I have experienced three types of this kind of missperception where visual clues and psychology play a big role:
An almost uniform slope but curvy road designed for cars. Obviously, when cycling you go on one side of the road. This, combined with the banked turns makes you feel like turns towards one side are much easier than opposite turns. Naturally the banking alters the actual slope angle at the sides of the road, but also the perspective of the road against the presumed vertical pines around it accentuates the difference between the expected effort and the actual effort of riding each next curve.
Another case is a short stretch of road that first goes down and then goes up. When traveling by car on that place, you feel that the first part is a steep descent and the second part is a steep ascent. But when traveling by bike, the "feel" is not as much. First in the descent you don't get as much "free speed" as you expect, so you "pedal hard" to help overcome the following ascent, but when you actually climb, you feel like if you where 2 or 3 gears lower than you are (effort wise). In this case, the lack of a visual reference of a horizontal line in the traveling direction fools the brain.
The third case is a coastal road that leads to a climb but before the beginning of the climb, there is a diversion that goes down to a boating club. Since both the main road and the diverted exit go along for a few meters, when you go over the main road you visually feel like you are ascending, but with very low effort. I.e. the brain is fooled into thinking the climb starts sooner then it actually does.
However, I have also experienced "temporal" delusions that Include wind effects, bike condition and physical condition.
Wind effects are pretty obvious. I know places where winds regularly change during the day, so you may have better luck climbing in the morning than late in the afternoon.
Sloppy maintenance on the bike can lead to more perceived effort, thus leading to feel a flat like it was a climb. At least in my case, I do my own bike maintenance, but sometimes I'm less careful and omit some revisions. So I take my bike (fooling myself) thinking it is in perfect working order while in reality, hubs, chain or derailleur pulleys needed deeper cleaning...
Finally, something that has happened to me a lot, is that riding a route, I feel pretty energetic the whole time, but the truth is I'm more tired by the end of it, so the sloped in the last part of the route are perceived steeper than they actually are. With flats it means you'd feel them like climbs. This can be proved with large looped routes. If you start in what usually is the middle point of the route, at first you will feel like the climbs have been flattened (respect to your memory of them) and at the end, you'll feel the opposite.