'Cool!' ...I think one of those Catherine wheels hit a rocket and it's like they mated, come and see!'
-- Seamus Finnegan (OP28)
Fred and George Weasley used their whole stock of their inventive fireworks to disrupt the school on Umbridge’s first day as Headmistress (OP28). Trying to make them vanish only multiplied them by ten each time. Cost was five Galleons for a Basic Blaze Box, and 20 for “Deflagration Deluxe.” The assortment included smoking dragons made of green and gold sparks, giant 5-foot Catherine Wheels like “flying saucers,” flying pigs of pink and silver, and sparklers that wrote swear words in the air (OP28).
Both Professor McGonagall and Professor Flitwick let Headmistress Umbridge get rid of the fireworks instead of doing it themselves, since she was always so concerned with using the proper "authority."
"whizbang" = a type of German artillery shell in World War I, named for the sound it made
Strangely, Umbridge didn't seem to understand that Filch was a Squib who couldn't do magic. As they watched the fireworks, she said "'Don't Stun them, Filch!" Instead, Filch grabbed a broom and swatted at them, which only set the broom on fire (perhaps a Wizard of Oz reference?).