I had friends who used this technique when they were living in NYC for a contract gig and one of the devs showed him.

Apparently the catch was you had to have a "two-trip" card. The other catch was these were not that readily available and were only distributed by specific social agencies. My buddy got one when he applied for SNAP benefits and said he needed the card for transport to his gig.

Once he had the card, he had his co-worker bend it for him and he said in the year he was working there, he never had to buy a metro card and saved quite a bit with the hack. Apparently it was still working as late as 2017 (The MTA never fixed the issue) but Metrocards were completely phased out last year as OMNY took over completely after starting in 2019.