OK, this happens to me almost every day. I start singing or humming a tune and, either because the melody is similar or maybe a key change suggests it, I finish the phrase with lyrics from another song. Here are two recent ones.
She goes running for the shelter of her mother's little helper, and I'll never have that recipe again!
Hold me Bat Boy, touch me Bat Boy, come on big baby come on!
Mother's Little Helper, Rolling Stones / MacArthur Park / Hold Me, Bat Boy (Bat Boy The Musical) / One Night Only (Dreamgirls)
"Do you know ChrisLovesShows?" "Yes. Why, yes he does!"