So I figured something out today.
I add and remove podcasts in iTunes all the time. And every so often, iTunes loses track of, er, tracks. This usually shows up in a smart playlist, where a podcast episode exists in iTunes’s database, but there’s no corresponding MP3 file on disk. You can’t manually delete entries from smart playlists; and if I’ve deleted the podcast, then I can’t even go back to the podcast list to delete the bogus episode. Nor does it show up in the “Music” list, since it’s a podcast episode.
What I finally figured out is that if you change the Media type from “Podcast” to “Music”, that’ll move the episode to the Music list, where you can delete it.
You can change the media type with “Get info” (⌘-I) → Options → “Media Kind”.
Unfortunately, if the file is missing, iTunes won’t let you edit the media kind. So first you need to associate the podcast episode with an MP3 file. The easiest way is to copy an existing MP3 file to /tmp/foo.mp3 or some such. Then, when you ⌘-I and iTunes says no such file and “Do you want to locate it?”, say yes, and point it at /tmp/foo.mp3. Then you can edit the media kind, and delete the file from the Music list.