The home side maintained a lead for about an hour, but their victory eventually slipped away.
Contractors' Alaa Kamal lived up to his word and showed 'how wrong Zamalek were by letting him go' this summer, and opened the scoring on 37 minutes.
Mohamed Samara doubled the lead a couple of minutes later to leave White Knights bemoaning.
Mahmoud Fathallah scored Zamalek's first goal two minutes after the hour mark, and Mosaad, who was sent in instead of Ahmed Gaafar, brought the score level in the dying minutes.
Zamalek are now in fifth place with four points, while Contractors lie eighth with three.
Zaki Exclusion
Many changes were made to Zamalek's line-up.
Striker Amr Zaki and midfielder Hassan Mostafa were excluded from the squad.
Both players were off-form in the previous game, which saw the Cairo side lose 2-1 to a stubborn Petrojet side.
Right back Ahmed Ghanem was also left out, following the return of influential wide man Hazem Emam from injury. The latter wasn't at his best though.
Towering striker Mido, who was also out-of-sorts in last week's game, sat on the bench and was introduced after the break. Gaafar played as a lone striker.
Ivorian midfielder Rémi Adiko made his competitive debut, which was far from impressive.
Left winger Sabri Rahil has been named in the starting eleven for the first time since the league got underway. Coach Michel Decastel was playing Mohamed Abdul-Shafi instead in the past three games.
Mohamed Abdul-Monsef stood between the sticks again, following his impressive performance against Petrojet.
Exciting Start
The match witnessed an exciting start, with both teams trying to notch an early opener.
Gaafar was close to opening the scoring after volleying Ahmed Magdi's cross from the left flank, but Mohamed Al-Akabawy blocked his effort.
Contractors replied right away after Rami Rabie released a precise low shot, which was competently saved by Abdul-Monsef.
Zamalek got the upper hand for a short while, but they collapsed all of a sudden to allow the Mountain Wolves to hit two goals in less than three minutes.
Energetic playmaker Kamal received Mahmoud Al-Sayed's pass unmarked at the edge of the area to slide home with ease.
Shortly after, Samara scored a carbon-copy goal, after taking advantage of another defensive lapse.
Firepower
After the break, Swiss boss Decastel introduced Mido instead of Gaafar, in an attempt to increase Zamalek's firepower.
The towering hitman made a quick impact by setting up Shikabala near the box, but the talented playmaker spurned his assist.
Zamalek took a stranglehold of the game and kept struggling to reduce the arrears, with the hosts' defense keeping Decastel's men at bay.
Eventually, Mahmoud Fathallah translated his team's dominance when he slotted home from close range after latching onto Ahmed Gaafar's headed assist.
Mosaad headed a last-gasp goal, his first after arriving at Zamalek from second-division Qanah in the close season, to save his club's blushes and earn them one point.