Struggling Chesterfield knocked Walsall off the Sky Bet League One summit with a hard-earned 2-1 win at the Banks's stadium.
The Spireites scored early in each half through Sylvan Ebanks-Blake and Ian Evatt before Matt Preston responded with an injury-time consolation.
It was the high-flying Saddlers' first defeat in seven league games but allowed Gillingham to steal a march at the top of the table.
To make matters worse they also had midfielder Sam Mantom stretchered off with a leg injury in the second half.
Chesterfield began brightly and deserved the lead they took in the fifth minute when Chris Herd delivered a deep cross from the right and former Wolves striker Ebanks-Blake, totally unmarked, headed firmly into the roof of the net.
It took Walsall some time to find their usual form, and as pressure mounted a 28th minute volley from Tom Bradshaw was saved at full stretch by Tommy Lee.
Four minutes later Lee managed to deflect a fierce drive from Mantom onto a post, then Reece Flanagan wasted a free-kick just outside the box on 42 minutes.
Chesterfield increased their lead in the 54th minute when big centre-back Evatt headed home a far post corner by Dan Gardner, and the visitors shrugged off heavy pressure as the game progressed.
Walsall would not be kept off the scoresheet, though, Preston opening his account for the Saddlers in the 93rd minute.