Tony Grant bagged a double as Glenavon stunned champions Linfield at Windsor Park on Saturday.
Paul Munster gave Linfield an early lead but Grant hit two goals in five minutes to put the visitors ahead.
Trevor Molloy and Gary Liggett struck just before the break to make it 4-1 and Linfield striker Glenn Ferguson hit a 53rd minute penalty over the bar.
Ferguson netted six minutes later to give Linfield hope but Glenavon held on while the Blues lost top spot.
Munster nodded in the opener from close range in the fifth minute after an error from Glenavon keeper Tudor Murphy.
Grant headed in a 22nd minute leveller and followed it up with a deflected shot past Alan Mannus.
Molloy's backpost finish and Liggett's lob over Mannus completed a remarkable half for the team which started the day second from bottom.
Ferguson wasted his penalty chance following Niall Hudson's foul on Michael Carvill.
The Linfield striker made amends when he volleyed home to reduce the deficit to two but there was no comeback as Stephen McBride's team claimed a memorable victory.
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