Two late goals from Peter Whittingham and Anthony Pilkington looked to have snatched a dramatic draw for hosts Cardiff, but substitute Ryan Williams' first goal in over two years won it for Barnsley in injury-time.
Paul Heckingbottom's visitors took a two-goal lead into half-time after Sam Winnall's double and Josh Scowen cancelled out Sean Morrison's early header, and following the late flurry of goals the South Yorkshire side were relieved to leave with a 4-3 victory.
Warnock's men could have grabbed the seventh goal, but Rickie Lambert sliced wide from close range before Williams proved more clinical at the other end.
Warnock said: "It was an exciting game for the neutral - on how not to defend! I'm disappointed with every goal. They were elementary errors.
"I've persevered for a few weeks but I've got to change it. I've had enough now, I have to be critical when I look at the goals."
With Cardiff trailing 2-1 in the first half, they were denied a penalty for what looked like a clear handball by Scowen as referee Tony Harrington waved the appeals away.
Just seconds later, Barnsley made it 3-1 after a rapid counter-attacking move saw Winnall net his ninth goal of the season.
Cardiff had 22 shots on Barnsley's goal, and Warnock was also astonished by some of the misses.
He added: "It was a cert handball. But the referee said he wasn't in a position to be 100 per cent sure.
"The chances we had, they take some missing. I'm really disappointed.
"I'm not going to name individuals but I'll be changing the team before the next game - and January can't come soon enough."
While Warnock considers transfer window business, Barnsley will look to build on their success.
Despite winning just their fourth game in 15, they find themselves six points from the play-offs.
But Barnsley boss Paul Heckingbottom is refusing to set a target for his players this season.
He said: "We don't set aims, if you set a goal you're limiting yourself and we just want to win as many games as we can.
"It's reflected in games like that. We've got a group of players who have responded to that over the past year or so. We know we won't win them all, but we'll try.
"I've had some big games in the short time I've been doing it. But in terms of the scoreline and nature of the game, it's probably the most dramatic game I've been involved in.
"It will be a good one for the fans to watch back, maybe not for us coaches."
Sporting Life