9. Rachel Needs Glasses, But Never Wears Them
The One Where Ross And Rachel...You Know - the fifteenth episode of season two - centres on Ross and Rachel going on their first date to the movies, with Rachel thereafter grovelling about the movie's subtitles. Apparently, this is due to her need for glasses, which she forbade herself to wear on their first proper date.
It's unfortunate for the writers, then, that this plot point was instantaneously abandoned. Not once throughout the ten seasons of Friends was Rachel seen wearing her specs, nor was it acknowledged why she required them. An uncomplicated one-liner could've been inserted to resolve why, exactly, Rachel has poor vision; explaining that she doesn't want to discuss it, for example, heavily implies a traumatic and/or distressing moment in her life, but it fixes the flaw regardless.
Friends superfans will be aware that it's impractical to think she'd be wearing contact lenses, as it was entrenched in season five's superlative The One With Joey's Big Break that she's frightened of putting anything in her eye. Though this provides viewers with a priceless credits scene where Monica, Phoebe, Ross, and Chandler pin Rachel to the ground to apply her eye drops, it remains a lackadaisical gap in Friends' story.