"Not another one!"
-- Minerva McGonagall (OP19)
Decree Twenty-five gave Umbridge the ability to overrule other teachers on matters of discipline (OP19).
Reasoning and implications
Once Umbridge had the power to overrule both Dumbledore and Professor McGonagall, she banned Harry and the Weasley twins from playing Quidditch “ever again” after they fought with Draco Malfoy after a match (OP19).
From the Web
Writing by J K Rowling on Pottermore: Dolores Umbridge
Pottermore feature: Dolores Umbridge
Harry Potter Wiki: Educational Decree Number Twenty-five