But the most egregious is something I've offhandedly mentioned here in the past. How the Main Title song from Mel Brooks' film, The 12 Chairs, is almost note-for-note Brahms' Hungarian Dance #4. Even down to the song's bridge. Yet the screen credit reads "Theme Song by Mel Brooks." I suspect that that phrasing was used so that he wouldn't have to say, not only "Words," but also "Music by Mel Brooks," when it's clearly not.
(The whimsy of this is that much of Brahms' Hungarian Dances are not by Brahms but rather his adaptation of old peasant folk songs.)
Anyway to refresh your memory, here's "Hope for the Best,, Expect the Worst." It starts a bit abruptly because there's a short prologue scene that comes before and was cut off. There are also subtitles added, so you can sing along. It's a wonderful song with very clever and funny words by Mel Brooks, and lively music by...well, you know.
And here below is Brahms' Hungarian Dance #4.
Often, it can be a little tricky determining for absolute certain whether a song was really, truly "based on" another. But I challenge anyone to say that "Hope for the Best, Expect the Worst" not only wasn't based on Brahms' Hungarian Dance #4, but isn't almost entirely (with maybe a tweak or two) this exact piece of music below, including the bridge.