Former Rotherham striker Tom Pope scored four times against his old employers to inspire Port Vale to a 6-2 victory.
Pope scored twice and was involved in the other two goals as Vale romped into a four-goal lead against the shell-shocked promotion favourites in the first 28 minutes.
Louis Dodds rolled the ball past Scott Shearer to open the scoring after fine play by Pope and Chris Shuker sent him racing clear in the fifth minute.
And Shuker was again the architect in the 16th minute when Pope headed his perfect cross inside Shearer's left-hand post.
Shearer couldn't prevent Pope adding his side's third in the 21st minute either as he looped the ball into the net after collecting Sam Morsy's pass.
Pope then picked out the run of Ashley Vincent following a Rotherham corner, and the fleet-footed winger sprinted half the length of the field before cracking in a left-foot shot from 20 yards.
Some of Rotherham's 750 travelling fans were on their way home when Michael O'Connor drove in a close-range shot from Ben Pringle's pass to reduce the arrears in the 31st minute.
And the visitors had hope for an improbably comeback in the 54th minute when Daniel Nardiello slotted a penalty past Chris Neal after referee Mick Russell had spotted a handball.
But Vale's victory was assured just after the hour-mark when poacher Pope tapped in after Shuker's free-kick was deflected against Shearer's left-hand post.
Pope then completed his and Vale's perfect day when he capitalised on some dismal defending to steer a cool left-foot shot beyond a stunned Shearer 12 minutes from time.