Crusaders saw their lead at the top of the Irish Premiership table cut to six points as their 13-match unbeaten run ended at Portadown.
Tim Mouncey's looping header five minutes from time gave the Ports victory in a disappointing match.
Second-placed Linfield beat Glentoran 2-1 at the Oval to cut the gap.
"It gives Linfield a lifeline and brings them into it a little bit, but I am not going to lose any sleep over this," said Crues boss Stephen Baxter.
"We have had a long unbeaten run and to lose this one is not the end of the world.
"Tonight was not good. I felt we were a bit nervy.
"We missed Paul Heatley's creativity and also Colin Coates."
Crues skipper Coates was suspended for the Shamrock Park match while winger Heatley was getting married.
Manager Baxter said Heatley would be available for Saturday's north Belfast derby clash with Cliftonville.
"Paul has been a talisman for us this season and we missed him tonight, but if we had won this match there would not have been a world about his wedding."