Draco Malfoy manages to bring Death Eaters into Hogwarts during his sixth year partly by using Peruvian Instant Darkness Powder from Weasleys' Wizard Wheezes:
"Anyway," Ginny went on, "he must have been checking whether the coast was clear to let the Death Eaters out, because the moment he saw us he threw something into the air and it all went pitch black-"
"Peruvian Instant Darkness Powder," said Ron bitterly. "Fred and George's. I'm going to be having a word with them about who they let buy their products."
Half-Blood Prince, Chapter 29, The Phoenix Lament
Ron seems to have a point. Fred and George weren't stupid. They knew they were in the middle of war in which the Malfoys were known antagonists. It seems strange that they would've sold Draco a product that was "handy if you want to make a quick escape" (chapter 6) in the light of the fact that:
- Arthur and Lucius had had a very public punch-up in Flourish & Blotts four years previously.
- Lucius had publicly called for the withdrawal of Arthur's Muggle Protection Act that same year.
- Draco has had a fierce rivalry with their brother for some years now.
- Lucius Malfoy at least was known to be a Death Eater to the Order (and so the Weasleys).
- Lucius had tried to kill both Ron and Ginny within the last few weeks/months!
- Did Draco or Narcissa buy the Powder on the day they were spotted in Diagon Alley during 'Draco's Detour'?
- Why did Fred and George not straight-up refuse to sell anything to Draco when he walked into their shop?
- If they did refuse him service, in what other way would he manage to procure the Powder?