There's one with a title along the lines of "The Dark Mark would clash with her pink cardigan" or something like that. It subverts the usual fanfic trope by having the scars discovered and reported make things so much worse. Umbridge and the Ministry spin it as Harry having done it to himself - after all, everybody knows he's a very disturbed child - and use it as justification to lock him away in Saint Mungo's.
1 Answer
A Dark Mark Wouldn't Match her Pink Cardie is what you're looking for.
Apart from the title matching pretty much exactly, the first few chapters set up the premise of the scars being found and everything getting worse from then on.
Harry shows Professor McGonagall the "I must not tell lies" scar on the back of his hand:
But suddenly words seem to have failed Harry. Not knowing what else to do, he undid the handkerchief around his hand and showed Professor McGonagall the words that would probably be there for the rest of his life.
Professor McGonagall goes to Fudge after finding out, and returns with the news that Harry is to be committed to St. Mungo's for self-harm.
McGonagall seemed to be having a hard time meeting his eyes. "It isn't a criminal hearing. It's with the Department of Social Care. Fudge is trying to make you a ward of the Ministry, and have you committed to St. Mungo's."
"St. Mungo's? For what? I feel fine."
"For self-harm."
Harry is then interrogated by Fudge and Umbridge, Harry insists that the scars are from Umbridge's punishments, Fudge and Umbridge insist Harry did it to himself. Harry ends up being sent to St Mungo's.