The bitterness of losing a derby can linger far beyond the season.
West Ham went a man down in the ninth minute the last time they met Millwall in 2012, but they still managed to claw out a 2-1 victory.
It would have left a scar on the Lions, who have seen their storming start to the Championship falter in the past few games, while their London rivals have leapt to the top following an unsteady start on their return to the Championship.
They meet at The Den on Saturday (12:30 BST) with the Hammers chasing a fifth straight victory and aiming to fully establish their position as league leaders.
Meanwhile, Molineux hosts its first Black Country derby since 2021 on Sunday (12:00 BST).
Wolves lost 3-2 in this fixture when West Bromwich Albion visited most recently but, like Millwall, it is 2012 that truly haunts them.
Baggies boss James Morrison played his hand in a 5-1 victory for Albion on the pitch that day and 14 years later he will be hoping to inflict further suffering from the dugout.