Mustard generally refers to any accidental error in an image.
The original Mustard was a small area of dark-grey (instead of black) pixels in the ground, appeared in frame , 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