Darren Boyce snatched a 90th-minute winner for Glentoran in a dramatic finish against Crusaders at Seaview.
All the meaningful action came in the second half as Glens skipper Colin Nixon headed in unmarked from Stephen Carson's cross.
Home captain Colin Coates powered in a header from a David McMaster corner in the 79th minute.
Boyce headed in from close-range after Jonny Taylor had nodded the ball forward in a frantic finish.
That sent the Glens players running to the touchline to celebrate with manager Scott Young but for the Crues it was a crushing blow.
Having finished second the Linfield last season and having held on to key players like Stuart Dallas and Jordan Owens, the north Belfast side were looking to get off to a good start.
After getting their equaliser, Stephen Baxter's men looked the more likely winners with Elliott Morris making a fine save from former Glens striker David Rainey and Coates had an effort cleared off the line by Richard Clarke.
When they were trailing, David McMaster hit the bar with a shot from distance and Morris got down to his left to palm away a shot from substitute Rainey.