These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
"El gran sarao" had the chapter where Filemón and the Super wrongly assume that Mortadelo has turned into a Serial Killer who decapitates anybody he encounters.
At the end of "La caja de los 10 cerrojos", Mortadelo, Filemón and the Súper throw a huge surprise box to the sea. In the next panel, it's revealed that the huge head that came out of the box contained an equally huge diamond inside, which was what the three men spent so much time working to find. Some fishes comment on how excited would be the surface inhabitants if they found it.
Ethnic Scrappy: Black people are usually portrayed as relatively sane, civilized and well articulate. However, they are drawn like in a 1930's cartoon. Chinese people are a different matter - They are both drawn, and act, like in 1930's cartoons! Even Chinese space program engineers are portrayed as buckteethed gnomes who talk (and eat) flied lice!
Black people tend to come in two varieties. Regularly-sized, relatively slender fellows who are particularly civilized. And big, muscular men who are easy to anger. An album taking place in New York City included both varieties. Attempts of Filemón and Mortadelo to investigate Harlem and locate a suspected terrorist, constantly end with them beaten by various locals who seem to have white guys as a target for their wrath. When the two agents finally get their suspect, he turns out to be a leader of the community. Their information about a bomb was wrong. His "bomb" was evidence about political corruption and how money supposedly going into urban development, ended up in the wrong hands.
This is mostly due of a case of Values Dissonance. Albeit still present, racism is not nearly as big in Spain as in other countries, so this portrayals are seen mostly as Played for Laughs, with no ulterior motives nor messages, conscious or otherwise.
"Funny Aneurysm" Moment: The current page image for this trope is a panel from 1992, in which a plane can be seen crashing into one of the Twin Towers of the World Trade Center.
One of their best stories is set in Germany, with M&F going all around the country in their mission (and successfully crossing the Berlin Wall twice!).
Interestingly, the parts where they cross the Berlin Wall were replaced with something else in the German edition.
And in Denmark (as Flip & Flop). Ibanez even made a story set in Copenhagen in honor of his Danish fans, featuring the Little Mermaid Statue as a main character.
Growing the Beard: The comic was an entertaining weekly strip with the traditional few-panels-and-punchline formula, but it wasn't until their first full book, Valor y al toro, that the now standard supporting cast appeared, allowing for deeper storylines and better interactions for the protagonist duo. Ibáñez's drawing style also evolved notably, imitating that of Hergé's Tintin.
Seasonal Rot: Most fans agree that it's been going on since The Eighties, though very occasionally a decent album still appears.