I've been playing blackjack off and on for 30 years. and it thought I had seen some pretty weird plays. But a few nights ago a player at my table had me shaking my head again and again.
From his small-talk and mannerisms, he obviously was not an amateur. He bought in for $200 ($15 min table) and 45 minutes later he cashed out for $775. (It was a fairly hot table, I took profits of $350 after about 90 minutes session.)
This was a 6-deck, 5/6 penetration, DDAS, resplit Aces, dealer hit S17 game.
He made many unconventional plays, but the one that boggled mind the most was him doubling down with a hand of 12, vs dealer 5 or 6.
He did this 4 or 5 times, and hit all of them. Not only did he not draw a 10-card, but the dealer did bust each time. I don't recall the exact True counts at the time he made these plays, but they were mostly positive because I recall I had larger than my minimum bet in place a couple of times.
I'm pretty sure he was counting on some level because his big bets were more or less in line with mine, using basic Hi-Low).