It's pretty obvious that one of the two pedals needs be tightened counterclockwise, but which one?
A brief reflection on the right pedal's sense of rotation (while pedaling forward) with respect to the crank would suggest that it is the pedal that needs to be tightened counterclockwise (so that it continues to be tightened further while pedaling).
It turns out that that's exactly wrong. Due to precession, it is the left pedal that needs to be tightened counterclockwise.
Likewise, with little thought and knowledge, a novice dabbling with bike mechanics, especially to fine tune derailleurs while riding, might be tempted to tighten a (brake or derailleur) cable by turning its barrel adjuster(s) clockwise.
Again, that's exactly wrong. Bowden cables are formed by a cable sheathed in a cable housing. The barrel adjusters neither tighten nor release the cable housing. They increase or decrease its length, and they affect the cable only indirectly—tightening the cable when the cable housing's length increases.
Hence to tighten, say, a derailleur cable to get it to a larger sprocket or chainring, it is necessary to release the barrel adjuster, which increases the length of the cable housing.
What are the counter-intuitive surprises an amateur bike mechanic encounters, after pedals and cables?
The question I meant to ask is precisely the one asked here, but clearly—and despite the two examples provided for pedals and for barrel adjusters—I used particularly poor wording that made the question so generic to be useless for learning with any depth.