This is "How Can I Explain?", which is a feature spotlighting inexplicable comic book plots, like, for instance, what happened to Falcon at the end of Captain America and the Falcon?
Now, due note that we get that a lot of these things are explainable outside the comic, but we're just talking about story elements that are inexplicable within the actual text of the stories. This is a perfect example of something that makes perfect sense outside the comic but that resulted in something with no in-story explanation.
Captain America and the Falcon was a series that was written by Christopher Priest in 2004-05 that dealt with Falcon seemingly becoming unhinged and resurrecting his "Snap Wilson" criminal personality. In the final story arc, Falcon's journey with a soldier designed to be, in effect, Anti-Captain America, resulted in the real Captain America getting shot and declared dead.
The final issue, though, reveals that the real Cap's death was a matter of the hospital instruments not being designed to measure a Super Soldier, so while a normal human would have been dead from his wounds, Steve Rogers is fine from his wounds. Falcon, though, has now snapped out of whatever personality issues he was going through due to the sight of seeing his best friend getting shot.
In the final issue (by Priest, Dan Jurgens and Tom Palmer), Cap goes to visit Robbie Robertson and they discuss how Falcon's actions might not have been as crazy as they originally seemed...
But Steve can't find his friend. Once Steve resolves the whole "Anti-Cap" storyline, he then thinks he sees the Falcon...
Now, in reality, this was designed to set up a Priest follow-up series starring the Falcon, but since that series did not happen, this is just completely unexplained.
The Falcon just shows up again in Ed Brubaker and Steve Epting's Captain America run like nothing happened...
That's truly it - this was just swept under the rug and it was done, never to be explained.
If anyone else has any other seemingly inexplicable comic book plots that you'd like to see us feature here, drop me a line at email@example.com!