"There was an odd feeling in the group now. It was as though they had just signed some kind of contract."
List of the DA members’ names, with their signatures.
At the formation meeting of the DA in Hogsmeade, Hermione asked the members to sign their names as an agreement that they'll keep the DA a secret (OP16). Unknowing to them, she jinxed the list (OP17); if anyone let the secret out, pustules spelling "SNEAK" would appear on their face. When Marietta confessed to Umbridge, her face was disfigured (OP27), and the pustules did not leave her face even the following year (HBP7)
When Ernie is hesitant about signing the list, Hermione reassures him by saying she won't leave the list lying around. But she does exactly that, by pinning it in the Room of Requirement. Amateur move, Hermione!