As writers we're supposed to know more about our stories than the words that make it onto the page. Can a character refer to something that happened, but happened off-page? As the writer, I know that it in fact happened, and there's nothing to negate the occurrence in the story (it did happen, but was not written into the draft.)
Example off the top of my head:
Betty thought, 'Mom used to talk about the mailman.' Betty says to the mailman, "My mom really likes you."
If Betty's mom talking about the mailman is not anywhere in the written story, on any page, is Betty's thought acceptable within the story? Or, do readers expect to have seen Mom talking about the mailman before Betty declares that it happened? Is it a shortcut to not have the event, simply a thought from Betty? Shoehorning the event into the story feels unnecessarily bulky.
(I feel like there must be a line here. Obviously, characters refer to all sorts of things all the time. I can't put my finger on what it is about this thought of Betty's that I'm unsure of.)