The opposite of "magnum opus"?

OED uses opus in eighteen definitions, and doesn't list a convenient phrase. (Ignore the red mark, that just positions the list)

Accessible list: opus; opus alexandrium; opus anglicanum; opus areanum; opus consutum; opus deista; opus filatorium; opus magnum; opus sectile; opus signinum.

Thesaurus.com doesn't list a specific antonym.

So: make one up. I don't know whether parvulum is widely understood, but it appears in some macaronic Christmas carols, and using a phrase which is similar to magnum opus might give the general idea.

parvulum opus — poor or small work

You might take a cue from theater reviews. There are so many epic failures (e.g., Damon Lindelof, Prometheus) that reviewers have to come up with new ways to describe them.

There are some relevant terms, though I don't know if any are as superlative as magnum opus. Possibly this is because we don't usually concern ourselves with relative degrees of failure or insignificance.

If you want a Latin word, opuscule is a good option. From Oxford dictionaries:

A small or minor literary or musical work.

That's very close to the opposite of a masterwork, and even has opus in it, but it's not at all a commonly used term (ODO notes that it is rare).

If you want a term that's more commonly used, the phrase in the definition above, minor work, is a good choice. Somewhat to my surprise, I couldn't find a good definition of the phrase, but it seems to me to be fairly common and has a transparent meaning. One representative usage:

King Stephen is one of Beethoven's minor works, and only its overture is performed with any frequency today.

—"Newly Discovered Beethoven Handwritten One-Page Manuscript Auctioned And Sold For $100K", Realty Today, Dec 22, 2015

Looking at the "culmination of life's work" aspect of magnum opus, juvenilia might be a good choice. The formal definition is simply

Works produced by an author or artist while still young.

But there is also often an implication that such works are, themselves, somewhat immature, produced before the artist had achieved full mastery of their art. It also has the benefits of being both Latin and at least somewhat familiar.

_{If you are open to more creative language uses, marginalia in it's extended sense of "nonessential items" (M-W) might be stretched even further to cover an artist's nonessential or marginal works, borrowing a bit of literary cred from its similarity in sounds to the above-mentioned juvenilia. Or for a very creative choice, I will note that my auto-correct keeps suggesting magnum oops.}