As a former B&M dealer, I've seen a lot (no one has seen
everything, but I've seen a lot).
I once had a player with a 20 (two face cards) on a $50 bet.
He put up an extra bet and told me 'double'.
I actually asked him three times if he meant to split (also a horrible idea) even though I wasn't supposed to question his decision.
He got angry and told me he knew what he was doing.
He was such a good player that he managed to tie the high score record of 30!
When dealing blackjack, if a player made what appeared to be well outside
their normal play, it wasn't correct to confirm the decision with them simply because the dealer isn't allowed to play the player's hand for them.
However...unless the player was a total asshole, I would 'pretend' to have missed their hand signal and I would make them repeat it. Usually they (or another player) would notice that they were about to do something mathematically stupid and I had very subtly saved the day.
Online, you can't quite expect that level of finesse, but you can expect that things like hitting on 20 are mistakes and the software should be designed to protect the player. It's excellent customer service and it promotes player loyalty to the casino.
When I play on Microgaming software, I have to go slower and be more careful simply because they have absolutely no player protection controls in the software. Additionally, the way they place their buttons is poorly designed which leads to plenty of accidental clicks if I'm not really attentive.