Basement boys Hungerford Town bagged a surprise win at Concord Rangers thanks to second-half goals from Taofiq Olomowewe and Liam Ferdinand.
The Crusaders got only their third win of the season to inflict a fifth straight defeat on Danny Scopes' men.
Bath City are up to eighth after a fine away win at St Albans.
Tom Smith scored the only goal of the game midway inside the first half to leave the Saints without a win in four.
It was a late late show from Chelmsford City after falling behind to Tonbridge Angels.
Khale Da Costa gave the home side the lead and that's how it stayed until second half stoppage time.
With the seconds ticking away, Chris Whelpdale grabbed an equaliser in the 92nd minute before Shaun Jeffers snatched all three points two minutes later for the Clarets.
Results elsewhere mean the Angels are now only within a point of the foot of the table while Chelmsford have the play-offs in their sights.
Saturday's National League South final scores:
Concord Rangers 0-2 Hungerford
St Albans 0-1 Bath City
Tonbridge Angels 1-2 Chelmsford