I started seeing missed steps in my prints. I haven't had this problem for a long time so I went through the basics of tuning up, lubricating moving parts, etc. The problem remained. I started printing test blocks and noticed the problem looked very regular.
This made me think it must be in the software tool chain and I spent a couple days trying to narrow it down. Nothing.
Finally, I took another look at hardware, even going so far as to disassemble things that have been working reliably for a while. Lo and behold, I found this:
These were some old gears that I printed when my quality was still pretty rough. I should have reprinted them a long time ago, but I forgot when they seemed to be reliable.
After you figure out a problem, it always seems so obvious.