Reasons why bugs might feel "impossible"

Hello! I’m very slowly working on writing a zine about debugging, so I asked on Twitter the other day:If you’ve run into a bug where it felt “impossible” to understand what was happening – what made it feel that way?Of course, bugs always happen for logical reasons, but I’ve definitely run into bugs that felt like they might be impossible for me to understand (until I figured them out!)I got about 400 responses, which I’ll try to summarize here. I’m not going to talk about how to deal with these various kinds of “impossible” bugs in this post, I’ll just try to classify them.Here are the categories I came up with for ways a bug might feel impossible to understand. Each one of them has a bunch of sub variants which are bolded below.