I blame tile arrangement. I only ever use tilesets when I really consider their layout easy to work with, and there's hardly anything that throws me off more than the official tilesets. Castle is a perfect example, with useful layer 4 tiles being placed on top and bottom of the tileset, split with background layers, foreground curtains, and those nearly useless slide tiles. Another offense they're guilty of is that you need to try really hard to make the most basic things tile correctly. Speaking of which, good luck with that in Carrotus; I spotted 3 subtle tilebugs in your picture so far (outside of the obviously unfinished bottom of the screen). I believe that reasonable amounts of Carrotus tilebugs are widely acceptable, just because they take ages to avoid - and because official levels also contain them - but no potential justification can speed up the wearisome process of reducing their numbers.

This is not a commonly explored field and I suspect it's often assumed to be a trait of bad design to create level sections where it's impossible not to get hurt; however, it can be pulled off relatively well. Deserto Orbital Base has the radioactive contamination area and I don't recall seeing anybody complain about that. The general rule is: place a full energy carrot before the challenge because you can't possibly know how much health the player got there with. Optionally add a checkpoint. You may also want to give the player a subtle hint that this is indeed the correct path to follow - climbing thorny vines might seem counter-intuitive. In the aforementioned example, EvilMike solves this by clearly leaving no other path and no way to backtrack, which is probably the best solution. Something as simple as an arrow sign might do but could instead give impression that you need to activate a trigger to get past. Meanwhile, a straightforward text sign would achieve the intended effect but get rid of most of the magic.