I finally realized that a frequent problem I encounter is caused directly by me.
Often, when I’m working on an ink drawing, I notice that there is a spot (or multiple spots) on the paper where the pen skips, or the ink beads up oddly. It’s quite annoying having areas where the paper doesn’t take the ink as well, and I have to keep working over the area to get it looking right.
The problem is most troublesome with dip-pens, but it’s also somewhat noticeable with the Pigma Micron pens I use.
I noticed these areas were primarily around the edges of the paper, but they sometimes cropped up in the middle of the page.
For a long time I suspected that oil from my skin was the problem, and that it was occurring in spots where I had touched the paper. But I mostly kept my hand resting on scrap paper while drawing so I wouldn’t smudge anything, so I wasn’t convinced that was the problem. Also, it wasn’t pervasive or serious enough for me to put a lot of serious thought into. Finally, though, on my last drawing, I ran into a few annoying spots, and decided I needed to figure out what was happening.
So I performed a very simple, obvious experiment that I should have done a long time ago: I lightly touched a spot on the edge of my paper, then started drawing around it. Result: There was poor inking in a small oval area that defined quite clearly the area I had touched with my fingertip.
So, it appears to me that at least with smooth-surfaced paper, pretty much any spot that comes into contact with my skin won’t take ink like it should. I’ve resolved to be extra-careful from now on not to touch the paper.
It’s not like before I was pressing on the page or laying my face down on it…I was only lightly brushing away grit, or letting the bottom of my palm touch when I wasn’t vigilant enough about sliding around my scrap paper shield. I just never realized that even a very light touch could cause me trouble!
Did everyone else already know this? Is everyone else reading this and saying, “Duh, glad you’re finally catching on!”?

Or am I just an especially oily character? (I wash my hands frequently, I swear!)
Edited to add: It’s not like I didn’t know that skin always, always has oil on it; and that oil always repels water. I think I had just, for some reason, had never consciously considered and accepted that very light handling from my hands was still enough to cause a very noticeable problem.