Using a tool from the ET GeoWizards toolbox. Error message says: "All the inputs are not current." Stopped outputs from going to in_memory and the error went away.
So I have an MXD with these green lines in the Page Layout view. They seem to serve no purpose, they might make the MXD render more slowly (though that is difficult to tell, as everything is so excruciatingly slow already), and most importantly, I want them to go away.
This solved it for me: "Draw a rectangle on the line(s), like if you're selecting elements. You won't see any visible indication that it's selected but that’s okay. Mouse right click on the white portion of the screen and choose Delete."