Mustard generally refers to any accidental error in an image. In an extension of the original meaning, on the One True Thread the term "mustard" has become a reference to any kind of mistake, malfunction or misfortune.
The original Mustard was a small area of dark-grey (instead of black) pixels in the ground, appeared in frame 1008, initially theorized to be mustard. It was later found to have been put there in error. The mustard, despite being a creation of Lord Randall, is generally regarded as heresy.
The mustard was first identified as such when Opiboble suggested that that the area of off-black pixels might be Gray-Poupon Mustard. The term was heartily agreed upon and has been used to this day. Its use has extended to include any slip-up which was retroactively corrected, such as the errant heads which appeared around frame 2231.