Counterproductive Propaganda

Exactly what it says on the tin, propaganda that is counter-productive to the goals of the group spreading it (or what should be the goals). A common version is to portraying the group’s enemies as stupid, weak, etc… than they are. It is common knowledge that underestimating your enemy is a terrible idea. So encouraging your own side to do this is also a terrible idea. This trope can and (let's face it) will result in those who are loyal to the group spreading the propaganda believing the propaganda, with catastrophic consequences. The survivors of the propaganda's consequences will usually realize how untrustworthy the apparent propaganda source is, and thus are likely to not use information they receive from that source. Which is somewhat a shame, in some cases, because sometimes that same source provides valuable information. Often, the propaganda infects even those that fabricated it.

Subtrope of Propaganda Machine. Compare Not Helping Your Case.

Literature

 * The Imperial Infantryman's Uplifting Primer has multiple examples, perhaps the biggest case of this is that it says genestealers are slow and sluggish and have blunt claws.
 * In Fifteen Hours propaganda states not only does the bureaucracy gets its directions from the God-Emperor of Mankind (that is allegedly infinitely wise), but that it passes on these directions flawlessly. This leads to spotted mistakes being ignored, because those mistakes were apparently intentional for whatever reason the observer can come up with. This, unsurprisingly, has catastrophic consequences.

Real Life

 * American propaganda portrayed the Japanese as stupid in WWII.
 * Both sides of the American Civil War were prone to this in the beginning. The Union propagandists that with their massive resources of men and resources they could easily overwhelm the South, while the Confederacy propagandists proclaimed they could just shock the Union into surrender because they were utterly convinced the Union's resistance would be token at best. And both sides were convinced the first serious amount of bloodshed on either side would cause the other side to realize the futility of their resistance, an illusion brutally shattered during the First Battle Of Bull Run/Manassas. After that point, both sides realized it wouldn't be that easy to win and changed their propaganda about their enemies accordingly.