The Treacle Tart issued a challenge that I couldn't resist

Well the characters are revolting... and pretty darn mad too. They have had it and are challenging you to justify yourself. The only real requirement is that you, as an author, and a character of one of your fics have a confrontation. No length restrictions or time frames. Frankly, I just want to know what the hell you were thinking. Please link to your original piece for reference and pimping, The Treacle Tart said. And yes, so I did:

Wherein Snape and Lupin pop by and cause Donna to turn into Donna-Sue