Of course, I may be too early, but here it is:
When I promised not to tell
I didn't know why not
But when I came up with "freckles"
I knew it had to stopI got caught by the talented
And wore a sign that said
That I was not to be trusted
Oh no, oh no, not me!
I think you are probably
Marietta Edgecombe, a minor character in Harry Potter and the Order of the Phoenix by J K Rowling.
This person
joined "Dumbledore's Army", a group of students meeting in secret. They all signed a piece of parchment promising not to tell outsiders about it. Hermione Granger had (without telling anyone) enchanted it so that, as she put it, "if anybody’s run off and told Umbridge [...] it’ll make Eloise Midgen’s acne look like a couple of cute freckles". What it actually does to Ms Edgecombe, boils appear on her face spelling out the word SNEAK ("a sign that said / That I was not to be trusted") and she is unsurprisingly mortified.
