Bradford produced their best display under new coach Francis Cummins to beat Huddersfield 43-18 on Sunday, while Hull KR defeated Warrington.
Stand-off Jarrod Sammut scored a hat-trick of tries as Bradford ended the Giants' 100% Super League record.
The Giants stay top of the table after second-placed Warrington lost at Hull KR but there was little else for the home fans to cheer after witnessing a woeful display by their team.
Huddersfield sorely lacked leadership and creativity in the absence of their skipper Danny Brough, who pulled out through illness, and never recovered from a shocking start in which they conceded two tries in the first nine minutes.
Hull KR produced their best 80-minute performance of the season to beat second-placed Warrington 26-12 in Sunday's clash at MS3 Craven Park.
Winger Dave Hodgson scored two of his side's three tries as the Wolves tasted their first defeat of the season.
Wigan, meanwhile, survived an almighty scare before maintaining their excellent recent record against Castleford Tigers with a tense 28-22 victory at the Wish Communications Stadium.
The Warriors came into this match having won seven of their last eight meetings with Castleford, the exception a 22-22 draw in June 2011.
On that occasion Wigan relinquished a 22-4 lead and they did likewise this time around as a wonderful score from Jamie Ellis and two from Justin Carney revived the hosts.
After Rangi Chase had struck the post for Castleford with a late drop-goal attempt, it was Chris Tuson who had the final say when taking Sean O'Loughlin's excellent offload to cross for his second try.
