I well remember, when I first returned to powered flying after a break of many years, being told off for allowing the Pa28 in which I was being instructed to drift up or down. The instructor put this down purely to my failing to position the control column appropriately to remain level while I was able, by reading the clouds and the landscape, to see that we were in fact flying through rising or sinking air-masses.
Ah but.... one of your jobs as a pilot instructed to fly at a given level is to compensate for rising and falling air as best you can to attain that consistent level, or as near to it as conditions allow.
I did, however, have a young instructor flummoxed one day in a PA38 of all things. I'd left my biennial too late to get my usual aeros guy for a session in our Chipmunk so did it with a school instructor in that Tomahawk. Not being aerobatic (the Tommy or the instructor) I decided some instrument flying practice would be a useful way to spend the hour.
Off we went, charts in the windows blocking my outside view, to do some climbs, descents, turns, combinations of these, etc, all with me on instruments. After a while I found I couldn't maintain straight and level. I had the correct power set, the correct speed trimmed, but we were going down. I pointed this out to the instructor and he couldn't explain it. So I removed the chart and had a look outside.
Upwind of us was a socking big North Wales hill. We were in lee-side sink. The instructor had never heard of it!