"...moldering, eldritch veils of such manifold depravity, the human eye will not process the sheer enormity of their malevolence, and out of self-preservation will merely show you paper bags with faces on them. These faces are still moderately scary, though, so brace yourselves for mild terror! [sound of bat wings]"
―Valve, on the hats' true forms
The Mildly Disturbing Halloween Mask is an unlockable headwear item wearable by all classes. It is a brown paper bag with a team-colored face painted on it, with each class having a different "face".
This item can only be worn around Halloween or during a Full Moon; at other times of the year, it does not appear in gameplay, unless the server has Halloween mode enabled. However, it can be equipped at any time and can still be viewed on the loadout screen even when it is not visible during games.