When everything is going right, as it has for the New York Yankees for much of the regular season, it makes little sense to take rash actions that could stir the pot. Apparently, Jorge Posada didn’t get that memo before he sparked what could be a costly brawl in Tuesday night’s game against the Blue Jays.

With Roy Halladay — who tossed a one-hitter against the Yankees on Sept. 4 — toeing the rubber for Toronto, Joe Girardi’s team knew that the contest would not be an easy one. It was made tougher by New York’s own starter, Sergio Mitre, who served up four homers and surrendered seven total runs in five-plus innings of work.

It was made even tougher when Mitre plunked Jays’ third baseman Edwin Encarnacion in the sixth, lighting a fire that — two innings later — would turn into an inferno.

In the top of the eighth inning, with Toronto leading 9-2, Yankees reliever Mark Melancon nailed second baseman Aaron Hill in the back with a first-pitch, 93 mph fastball. The hard-throwing rookie right-hander had already hit three batters in 15 innings entering the game, so he’s certainly no stranger to the HBP — malicious or otherwise. The fire began to spread.

As southpaw Jesse Carlson returned to the mound for a second inning of work, the drillings were on his mind. Looking for revenge to protect his teammates, with one out in the bottom of the eighth, Carlson opened Posada’s plate appearance with a fastball behind the Yankees’ catcher. A brief clearing of the benches followed, and an exchange between the players led to warnings from home-plate umpire Jim Joyce. But it appeared that order had been restored.

Except, Carlson walked Posada, and Posada later came around to score on a double by Brett Gardner. As Posada touched home plate, he raised his elbow and bumped Carlson, who was backing up the play but standing closer to the plate than a pitcher normally would, perhaps hoping to rekindle the flame. If that was Carlson’s goal, he certainly achieved it.

Joyce immediately ejected Posada for the incident, but Carlson kept chirping, and seconds later, both teams’ full rosters were back on the field for a full-fledged brouhaha.

Carlson, Posada, and Jays catcher Rod Barajas were the most active participants, but even Girardi ended up in the middle of the fisticuffs, and umpire Joyce only narrowly escaped. Girardi left with a few cuts and bruises on his head, and Carlson got his comeuppance — an ejection, along with a massive welt on the left side of his forehead.

Things settled down after the brawl, enabling the Jays to complete a 10-4 win, but the fallout from the fight is far from over. Posada and Carlson likely will receive hefty fines and lengthy suspensions for their actions, and other players also could be reprimanded.

Losing Carlson’s arm from the relief corps for a few days won’t be a huge hit to the Jays, and the Yankees — given their cushy 6 1/2-game lead over Boston in the AL East race — should be fine with Jose Molina or Francisco Cervelli getting some extra time behind the plate. 

But as the playoffs draw near and tensions start to rise, players can’t afford to make mental mistakes. For a veteran and team leader like Posada, inciting a brawl that could have resulted in a serious injury to a key player at a crucial moment in the season certainly qualifies as a bad decision.

This was a much different brand of mental mistake than losing track of the count, as Posada did in an at-bat against the Baltimore Orioles two weeks ago.

The Yankees — taking advantage of the perks of a cakewalk into the postseason — were able to laugh casually about that gaffe. Posada’s actions on Tuesday, on the other hand, are no laughing matter.