Wigan Warriors slipped a position to 5th after the completion of round 20 today.
The Warriors climbed to 4th on Friday when they beat Salford 28-12.
But Castleford's 42-10 win against London Broncos this afternoon drew them level on points - and the huge margin of their win managed to overhaul the Warriors' superior points difference. They are on +20, with Wigan on +17.
There was some good news for the Warriors, though.
Catalans went down 31-12 at Leeds, surrendering the chance to move into 4th and two points clear of the chasing pack.
Instead, the 6th-placed Dragons remain level on 20 points with Cas (4th) and Wigan (5th). Salford, in 7th, have 18 points.
St Helens and Warrington lead the way with nine rounds to go before the play-offs, with 36 and 28 points respectively.
Hull FC are 3rd with 24 points and the Warriors will be able to cut that margin to just two points if they beat Hull KR on Friday and FC lose to Saints.
At the bottom, Leeds' win moves them level with London, Hull KR and Huddersfield.
All four sides - a third of the competition - are on 14 points, with the Broncos bottom because of their poorer for-and-against.
In the Championship, Wigan's Jack Wells scored one of Swinton's tries in a thrilling 36-28 win against Rochdale.
The Warriors' Craig Mullen, Liam Byrne and Samy Kibula also played for the Lions on either loan or dual-registration, while centre James Worthington was in the opposition ranks - in a game which was controversial allowed to be played at amateur outfit Rochdale Mayfield's home.