Based on a list started by NLC, with input from nonanonnotcow and gold-digging nanny, here is the list of Mallard Fillmore comment codes, for easy future reference:
1 - Has completely misquoted his "source".
2 - Has completely fabricated his fact/quote.
3 - Has utterly misunderstood the technical issue/science issue that he sites.
4 - Has clearly misrepresented the technical issue/science issue on purpose.
5 - Has used the same gag three days running.
6 - Has used the same gag four days running.
7 - Has used the same gag five days running.
8 - Has used the same gag six days running.
9 - Has used the same gag for a whole week or more.
10 - A character has his/her "speech balloon" coming out of an inappropriate part of his body.
11 - A character is drawn with one or more penis-shaped body part(s).
12 - Unrecognizable caricature.
13 - The bad liberal/union representative/feminist/etc is drawn as utterly physically repulsive.
14 - Simply retelling of a "blonde" joke using a different stereotype.
15 - Simple retread of "Joke #117 from the old-gags book".
16 - Self-aggrandizing (e.g. references to "all the letters he got", or the "his grass roots campaign" for some unknown politician).
17 - Unreferenced repetition of an obvious urban legend.
18 - Unreadable "foot note".
19 - Out-of-context misrepresentation of material in footnote.
20 - Depiction of opponent as defending or supporting some unspeakable monster (e.g. Pol Pot or Stalin).
21 - Straw Man - Use of quotation with implication that anyone actually holds some stupid or reprehensible point of view.
22 - Absolutely no idea what his point is.
23 - Mercifully, no politics in today's strip --but even less funny than usual.
24 - Lazy and only used one panel on a weekday.
25 - Lazy and only used one panel on a Sunday.
26 - Overuse of four-dot ellipses.
27 - Too many words.
28 - Has cited an obviously biased news source as his only support.
29 - Has cited an entire publication/site as his source (e.g., USA Today or Google).
30 - Demonstrates Extreme Brazenness or Hypocrisy regarding Republicans (IOKIYAR).
31 - Uncritical Regurgitation of Republican Talking Points.
32 - Creepy Unintentional Self-Revelation.
33 - Includes a Seriously Disturbing Image.
Despite the obvious advantage of some re-organization, I maintained the order of the original list and additions, to maintain backward-compatibility with past comments.
Feel free to suggest additions to the list!