In "A Very Old Man with Enormous Wings," what fixed ideas is the author asking the reader to question?
One fixed idea about angels is that they are spiritual creatures, direct messengers from God who exist to bring messages and morals to the human race. In the story, the angel -- if it is actually an angel -- is old, dirty, and incomprehensible. Any message it might bring is overpowered by its obvious Earthly faults: it smells, it can't communicate, and it can't perform miracles on demand.
...the few miracles attributed to the angel showed a certain mental disorder, like the blind man who didn't recover his sight but grew three new teeth, or the paralytic who didn't get to walk but almost won the lottery, and the leper whose sores sprouted sunflowers.
(Márquez, "A Very Old Man with Enormous Wings," salvoblue.homestead.com)
All those "miracles" could be simple coincidence, only thought of as miracles because they occurred soon after meeting the angel. Even the ridiculous idea of "sores sprouting sunflowers" is reasonable considering that this story contains a rational woman whose body is that of a giant spider. By showing the angel as a dirty, non-spiritual creature, the author challenges the idea that supernatural creatures might be understandable on the human level of intelligence and reasoning; instead, a supernatural creature might actually be completely alien to humans, impossible to communicate with, and without any frame of common reference.