Last year there had been seven dismissals by this stage of the season, so the fact referees had been able to keep their red cards in their pockets was a huge boost to the sport and its safety record.
Official Luke Moore had no option however, as Cust, clearly fired up by the occasion in front of Hull FC's biggest attendance for 15 years, shoulder-barged Mourgue's head as the France full-back fielded a kick behind the posts.
At that point the Robins were already in front as Mikey lewis, back from injury, showed his magical qualities with a sharp break and slick pass that sent Gildart running clear.
With more than three-quarters of the game left Cust's rush of blood gave KR a huge advantage and they used it as Joe Burgess sent Hiku over after an overload down their left.
Ashworth gave the home fans in the sell-out crowd something to cheer as he sprinted to grab Aidan Sezer's grubber kick before two defenders, but Mourgue, back from a head injury assessment following Cust's challenge, scampered over to give his side a 12-point half-time lead.
The Black and Whites were clearly fired up in the second half and threatened to produce one of the great comebacks as they threw the ball around with brilliant abandon, with Rapana setting up Briscoe for an excellent take and finish.
Hull FC even had two successful Captain's Challenges as the video ref spotted KR players ripping the ball out, and from the second the pumped-up home side went within two points, as Rapana finished another fast passing move to swerve inside and score.
KR looked rattled and the home crowd was bouncing, but Hull FC's efforts soon began to tell and Rovers pounced as their opponents tired.
They responded with some champagne rugby of their own, keeping the ball alive, with James Batchelor, back from injury, carving inside to dash over and extend the lead.
Waerea-Hargreaves, due to retire at the end of the season, capped off the win by grabbing the ball and providing an aerial finish after Hiku had knocked back Lewis' high kick.
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