Castleford's hopes of finishing the Women's Super League season with a 100% record were ended by St Helens in the final match of their campaign.
Tigers were on course to complete their perfect record when they led 10-4 at half-time through tries from England duo Shona Hoyle and Tara Stanley.
But second-half scores from Jodie Cunningham, Katie-May Williams and Tara Jones saw Saints run out 20-14 winners.
The two sides could meet again in the Women's Grand Final on 11 October.
The game was switched from the Totally Wicked Stadium, which will stage the women's showpiece, to the home of a nearby community club because of heavy rain.
And although St Helens' victory did nothing to change the final league standings or the teams' fixtures in the first round of the play-offs on Sunday, they will hope to have sowed a few seeds of doubt in Castleford minds should they meet again a week on Friday.
Cas will host the 2018 champions Wigan in the semi-finals, while Saints take on Leeds Rhinos, who beat Tigers in the Challenge Cup final in July.
In Sunday's other games, Leeds warmed up for the play-offs more positively than Wigan by shutting out the champions 16-0, courtesy of tries by Courtney Hill, Abby Eatock and Keara Bennett.
Featherstone Rovers ended the season in style by beating Wakefield Trinity for a fourth-straight win to secure fifth place above Bradford Bulls, the inaugural WSL champions in 2017.
St Helens 20-14 Castleford Tigers
Leeds Rhinos 16-0 Wigan Warriors
Bradford Bulls P-P York City Knights
Featherstone 24-0 Wakefield Trinity