Each day in June you'll get an entry showing you the first appearance of seemingly minor characters, phrases, objects or events that later became notable parts of comic book lore. Not major stuff like "the first appearance of Superman," but rather, "the first time someone said, 'Avengers Assemble!'" or "the first appearance of Batman's giant penny" or "the first appearance of Alfred Pennyworth" or "the first time Spider-Man's face was shown half-Spidey/half-Peter." Stuff like that. Here is an archive of what I've featured so far.
First Appearance of S.T.A.R. Labs
As per commenter Dominic's request, let's take a look at December 1971's Superman #246, where writer Len Wein and artists Curt Swan and Murphy Anderson first took us to S.T.A.R. Labs...
First time Wolverine made the "snikt!" noise with his claws
"Snikt!" must be one of the most famous sound effects in comic book history.
When Wolverine first showed up, his claws did not pop out (they might have been designed to do so, but they did not actually ever do so in the issue).
So it was in his second full appearance, in 1975's Giant Size X-Men #1, that Wolverine both first "popped a claw" and we first saw that oh so special sound effect...
First time we met a living relative of Bruce Wayne
In 1954, we met the first living relative of Bruce Wayne, Aunt Agatha, in Batman #89 in a story by Bill Finger, Sheldon Moldoff and Stan Kaye...