I
searched all downloaded posts (which took hours) for anything that could be a Bitcoin private key. That resulted in 9375 potential keys (not all of them are valid, and no, I won't post the list).
I think that the number is bigger, since some have posted them as images which you could not "scan".
Even more, in some cases I remember there was a scam spree of posting "by mistake" ETH private keys for wallets containing tokens and no ETH. Obviously, at funding a smart contract was sending the ETH away.
So it was not only mistakes.
Yesterday, I imported the private keys into a new Bitcoin Core wallet (this took only a few minutes), and did a rescan (which took forever, but was mesmerizing to watch: the balance went up and down by many Bitcoins, and this kept going for hours! I left it to finish overnight.
This morning, Bitcoin Core was hanging. I killed it, restarted it, and it took forever to load the wallet (which had grown to 2.2 GB during the rescan). Eventually, it worked!
Impressive! I've intended a similar test some years ago with Electrum (my set was much smaller and I don't even remember if I've done my test with keys or just addresses). But for some addresses the number of transactions was too big and the Electrum servers were cutting me off (I knew less back then). So I've abandoned the idea of watching those. However, I've noticed even back then that even if some addresses were known to be leaked and unsafe, some people still were playing with them long afterwards (by funding with small amounts). But I was not aware of huge mistakes like that 0.84BTC... wow...