A furious Max Verstappen deliberately drove in front of Lewis Hamilton as the one-time rivals clashed in practice for Sunday's Emilia Romagna Grand Prix.
Verstappen, who was aggrieved that Hamilton had obstructed him on a flying lap, launched his Red Bull into the Mercedes driver's path here in Imola.
Although Verstappen made sure to avoid making any contact with Hamilton's machine on the exit of the Villeneuve chicane, the championship leader slowed down to make his feelings clear by taking his left hand off the steering wheel and gesticulating in Hamilton's direction.
Hamilton accepted blame for getting in Verstappen's way, and acknowledged the error by raising an apologetic hand in the direction of the Red Bull man.
The flashpoint summed up an erratic day in Imola for Hamilton and Verstappen, with both drivers making a series of uncharacteristic mistakes in the two practice sessions.
"My God," yelled Verstappen over the radio, moments before his altercation with Hamilton. "I don't know, man. It's so difficult, everything. This time suddenly the front grips up a lot, I almost spin."
The Dutch driver finished only seventh, half-a-second behind Charles Leclerc, who raised hope of a Ferrari win on home soil after he topped both sessions.
Leclerc, in his upgraded Ferrari, was 0.192 seconds clear of McLaren's Oscar Piastri with Yuki Tsunoda a surprise third for RB.
Mercedes have also brought a series of revisions to the seventh round, and Hamilton finished an encouraging fourth, 0.391secs back and one place ahead of team-mate George Russell.
Prior to his run-in with Verstappen, Hamilton also irked another former rival, Fernando Alonso. Like Verstappen, the 42-year-old accused Hamilton of getting in his way.
"Hamilton thinks that he is alone on the race track," said Alonso. "It is the second lap he does it, turn 12 and now turn seven."
In first practice, Verstappen had been on course to set the fastest time of the day so far in his Red Bull, only to lose the rear of his car on entry to the chicane at turns 14 and 15.
Just moments later, the Dutch driver was on another hot lap before he ran through the sandtrap on the exit of turn 11.
Earlier, Ollie Bearman was back in an F1 car for the first time since he made his debut as a last-minute stand-in for Carlos Sainz in Saudi Arabia in March.
The 19-year-old, who is set to land a full-time drive with Haas next year, was handed a practice outing by the American team, finishing 15th, 1.6secs behind Leclerc.
McLaren's Lando Norris, who broke his duck at the Miami Grand Prix a fortnight ago, ended the day in 12th.
