The historian Doukas gave an account that a small gate called the Kerkoporta was accidently left open by a team of soldiers that would venture beyond the walls and carry out attacks against the Ottomans. This resulted in ~50 Ottoman soldiers gaining entry to city and raising their banner on top of the Inner Wall causing panic and retreat by the defenders.

While this is credited as the blunder that caused the Fall of Constantinople, the walls had been battered to pieces and the Ottomans had been able to breach and enter the city only to be pushed back by the defenders led by the experienced Genoese soldiers. Once the Genoese commander Giovanni Giustiniani was wounded and pulled back from the wall the Genoese began to pack it up and the Byzantines became unorganized without the strong leadership Giustiniani provided. Kerkoporta or no kerkoporta, it was just a matter of time until the Ottomans swept into the city.

Kazhdan, Alexander, ed. (1991). Oxford Dictionary of Byzantium. Oxford University Press.