This is Past Was Close Behind, a feature that spotlights moments, exchanges, etc. from older comics that take on a brand new light when read in concert with later comic books or events. Basically, stuff that looks hilarious in hindsight.
One of the all-time oddest members of the Avengers was Demolition Man, who went by the shortened title, D-Man. He was a vigilante who befriended Captain America during the time period where Steve Rogers had lost the title of Captain America and so was being a superhero as The Captain.
D-Man became one of Captain's closest allies, and so when Cap decided to re-form the Avengers after Inferno, he naturally chose his friend, D-Man, to help join the team. However, tragically D-Man was seemingly lost during battle in the arctic. When he was discovered 50 issues later (by USAgent and Falcon, as Cap was off in space as part of Opetation: Galactic Storm), his brain was muddled from the time spent in the freezing arctic.
When he recovered, he became a new champion of the homeless, living with them as their own personal superhero. D-Man never actually appeared in an issue of Avengers back in the day.
However, when Kurt Busiek, George Perez and Al Vey re-launched the Avengers in 1998, the opening story arc involved calling in EVERY Avenger, including guys who never actually joined the team, like D-Man...
Busiek played D-Man as a bit of a comic relief character (not because HE was a joke, but just that other heroes had a hard time imagining that this guy who is a homeless superhero is actually a real Avenger).
In Avengers #1, some of the other Avengers mocked the idea of D-Man being a real Avenger, and Namor, who is famously a bit of a jerk, made a comment that D-Man did not really fit as an Avenger and that perhaps he should look into joining the X-Men, as they are apparently (according to Namor) much less choosy about their members...
The reason that this is funny is that, sure enough, less than 10 years later, Namor ended up joining the X-Men himself!
Namor, basically, did a reverse of the old joke, "I'd never belong to a club that would have ME as a member." Namor instead mocked the idea of people joining the X-Men and yet ended up on the team anyways. Well, at least Namor IS a mutant, so he's got THAT going for him.
If anyone else has a suggestion for some hilarious in hindsight stuff, let me know by dropping me a line at firstname.lastname@example.org!