The Belgian goalkeeper might have been culpable for Papiss Cisse's goal.
The Reds looked in cruise control against the relegation-threatened outfit as goals from Daniel Sturridge - who netted inside 90 seconds - and Adam Lallana helped the hosts into an ever so comfortable 2-0 half-time lead.
But within minutes of the restart, the Magpies were thrown a lifeline when striker Papiss Cisse headed into an empty net, Jack Colback completed the comeback on 66 minutes to win a big point for United in their continued fight against relegation.
It was Newcastle's first goal which puzzled Sinclair, suggesting that the perceived error by Mignolet, who came to parry a cross but missed it, allowing Cisse to nod home, played a big role in Rafael Benitez's troops leaving Merseyside with a point.
He said on BBC Sport: "As a winger it is one of them where you think, 'Don’t come for that, keeper.’ Mignolet can’t come for it, he got nowhere near it and it is a simple header for Cisse."
Liverpool fans ripped into Mignolet on Twitter, given that it was certainly not the big Belgian's first error in a Liverpool shirt and manager Jurgen Klopp might well replace, or least sign decent competition for him, in the summer.