Zinedine Zidane's son Luca was among two Castilla players sent off as Real Madrid's youth side suffered another defeat on Sunday.
Santiago Solari's side were 1-0 up against local minnows Navalcarnero early in the second half, although with 10 men, when goalkeeper Luca Zidane dwelt on the ball after receiving a back pass, lost possession and hauled down the opposition attacker.
Referee Juan Luis Pulido Santana showed another straight red card, the penalty was converted, and Castilla's nine men eventually lost 2-1, meaning promotion from the third tier is already looking difficult.
Castilla now sit 12th in Segunda Division B Group 1, with seven points from their first six games. The division is currently topped by Celta Vigo B, with Deportivo La Coruna's youth team in second spot and Atletico Madrid's youngsters in fifth.
The first Castilla player to be sent off in the game was Oscar Rodriguez, 19, who scored a stunning long-range goal in Madrid's International Champions Cup game against Manchester City in Los Angeles in July.
Luca Zidane, 19, is playing week to week with Castilla, while also filling the role of the senior squad's third-choice goalkeeper.
When first-choice Keylor Navas was rested following the recent international break, the France youth international was on the bench as backup to starter Kiko Casilla.
Ex-Madrid player Solari kept his job last summer despite Castilla's failure to even make the promotion playoffs, although there had been speculation that his former teammate Guti might replace him following his success with the club's under-18 team.
Castilla's failure to gain promotion from the third tier in recent seasons, including with Zinedine Zidane in charge, has seen the club place promising youngsters on loan at other clubs so they can gain experience at a higher level.
Martin Odegaard is currently on loan at Dutch Eredivisie side Heerenveen, Fede Valverde is in the Spanish top flight with Deportivo's senior side, while Philipp Lienhart joined Bundesliga outfit Freiburg in the summer.