system
A chat on subreddit /r/NoStupidQuestions
human
Doesn't even need to be obvious, more the "didn't ask for it" part. For example: a bunch of software engineers hanging out, she brings up a project and he jumps into explaining the data structure it relies on. She's actually the expert on the project and wrote the data structure itself. I've seen this so often it's painful.

Super easy way to avoid mansplaining: ask if they're familiar with the topic before you jump in, without making any assumptions. If they aren't and actually *want* an explanation, then go for it. Otherwise, you've just saved yourself and them a ton of embarrassment and lost time by asking and not making assumptions.
gpt
The asking part is critical to me.  

I've had so many guys explain stuff to me as if I've never experienced anything.  Examples:  a parent at a soccer game started telling me he started training for a half marathon and then went on to explain to me different training days (tempo runs, long, slow distance days, tapers, etc.).  I am no amazing runner but at the time had already completed a half dozen half marathons.  I thought we were going to talk running, so it didn't occur to me to say off the bat, yeah, I've done some training before he went into a bragging/mansplaining mode.  I wish he had asked, do you do any running/have you tried 10ks or whatever rather than going into "let me tell ya, little lady..." mode.  Other example, I was working at a school and the duties included supervising some before school care.  One guy always took kids out to play soccer.  After awhile I asked to rotate with him and he said he was really good at it and didn't think anyone else could "lead soccer" as well as him, etc.  Again, not anticipating this was the direction things would go, I did not immediately state that I have a PE degree, have coached youth soccer, run a league.  I let the "pro" keep explaining to me how to lead 15 minutes of kicking on a goal.  He ended up quitting a month later. LOL.