One of the most famous lyrics in Eminem’s discography—the one that Dave Chappelle famously parodied—was apparently as ad-libbed is it seems to have been. Eminem recently annotated the lyrics to “Cleanin’ Out My Closet” on Genius and confirmed as much.
Regarding the opening spoken line, “Where’s my snare? I have no snare in my headphones,” Eminem wrote, “This was true. I was recording and couldn’t hear the snare, the engineer had it muted for some reason. I left it.”