Blackface. You just don't do it, white people.
On the Thursday edition of "Late Night," Seth Meyers went through some important dos and don'ts regarding Halloween costume choices. For example, DO dress weather appropriate and DO make your costume topical. DON'T dress in blackface.
Check out the "Late Night with Seth Meyers" clip above and understand that this really can't be stressed enough: If you're a white person dressing in blackface, your intelligence is seriously in question. Just don't do it.