purchase [some Beatles ]
As the most recent notice to SMM bloggers said, the traditional leftovers theme could be interpreted several ways: missed opportunity songs you might have posted during the past 12 months' themes, or perhaps something you can relate to leftovers of another sort.
Myself, I confess that I have to push to meet the bi-weekly output of SMM, and although I have discarded a number of posts over the past 12 months, there are none that I am driven to return to as leftovers that need re-working and then posting.
On the other hand, there are various Beatles songs that didn't make the official cut, and I guess they qualify as a kind of leftover. While you can find out-takes and un-published studio sessions for most every musician, a look back 50 or so years in and of itself amounts to a leftover of sorts. Yes, it was more or less "50 years ago today ..."
There is an extensive Wiki article about bootlegs of Beatles recordings: considering the number of performances - both in semi-private venues as well as over the airwaves, it is no surprise that there exist unofficial copies of performances (despite the limitations of the days' technology - If you are younger than 50, you ought to take the time to research what it was like to make a copy of live/radio ... music before the 90s.)
A decade or so ago, I ran across the original Beatles Complete on Ukulele project at which time they were providing a download link to each of the songs. The songs are still online but not longer easy to download. The site notes that they have a version that includes a ukulele of each of the 185 officially released Beatles songs.
Lennon/McCartney were prolific composers, and there are a number of songs that weren't distributed by their various recording companies: kind of left overs. Two in particular here from different eras of their careers The Palace of the King of the Birds from about the Magical Mystery Tour days, and then Bad to Me from 1963: