Sort of "accidental" plagiarism should be really uncommon; especially if the statement was something really really common.
For example: "A hardware wallet is the best option for holding your cryptocurrencies long-term". While I literally just wrote that myself right now with no intent of copying anyone, I wouldn't be surprised if the same exact line was posted at least once in the past on some article or forum post. Obviously this shouldn't be bannable simply because it's very common information.
That example by mk4 is plausible, because there are generic types of statements we've read that we might rewrite from memory without realizing it, but those IMO would tend to be very short statements and could be "mistakes". That's the only scenario in which I can see someone making a good argument that they didn't plagiarize something.
But in general, I think that 99.99% of plagiarism is not done mistakenly. But what have we learned from all the ban appeal threads when someone gets busted for plagiarism? They claim it was a "mistake". So it doesn't surprise me that Mpamaegbu is making that argument (though I don't know the details behind it). If it only happened once and it was one of those sentences that anybody could write, then maybe it was unintentional--we'll see once all the facts are in. Most of the time, though, there's no mistake at all. The member caught copy/pasting really did do that.