Here's an example from a movie that people have actually seen.
He was totally right, too. These characters show up famously in movies like Garden State, Sweet November, and Enchanted. But my favorite example is probably from Doctor Who, in which longtime companion Clara Oswald actually says: "I don't know where I am. It's like I'm breaking into a million pieces, and there's one thing I remember. I have to save The Doctor." It's important to have a purpose in life, I guess.
"Look at all my agencyyyyyyyy!!"
It was a neat observation. And why it's an issue was actually summed up pretty perfectly by Natalie Portman herself:
"I appreciate that people are writing characters that are interesting and unusual, rather than some bland female character as the girlfriend in a movie, but when the point of the character in this movie is to, like, help the guy have his arc, that's sort of the problem, and that's why it's good that they're talking about it, because it certainly is a troubling trope."
There's just one problem: The Internet is super dumb.
How We Ruined It
The biggest misunderstanding surrounding Rabin's criticism is that it describes any kooky female character. That's why Diane Keaton in Annie Hall and Katharine Hepburn in Bringing Up Baby and Maria Rainer in The Sound of Music have been called "Manic Pixie Dream Girls." You may as well say that Sally Albright from When Harry Met Sally is one, too. And Marge Gunderson from Fargo, because she's got a funny accent. Or you could acknowledge that giving a female character some distinguishing traits is actually just what writing is.