Barcelona B came from one goal down to win 2-1 on Sunday against Castellón, who suffered their first defeat in the league this season at the Johan Cruyff stadium.
Raúl Alcaina's goal gave the visitors the lead and they looked on track to keep top spot before Barça's youngsters turned things around late on.
Defender Ronald Araujo drew the hosts level in the 83rd minute and Monchu kept his cool to score from the penalty spot in the 88th minute to earn all three points.
The result allowed Sabadell to jump to the top of the table, while Barça moved up to fifth, just one point back from Lleida in the final playoff spot.