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
we had this at my old job and it was hilarious. people would try to explain to her how thing X worked, not realizing she was actually fairly well known for having kind of pioneered the whole topic.