There are various fragments of Greek and Latin poetry whose provenance is unknown, such as quotations in other works whose source hasn't survived.
Are there any known fragments longer than a couple lines, whose author is unknown or uncertain?
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Authorship and the rights associated with it, as we know it now, is a relatively modern concept. In antiquity, works had a higher probability of being preserved (by being distributed more, by being sold more, by being in higher demand) if they were ascribed to a famous author, preferably a dead one who couldn't disclaim authorship anymore. Thus "book sellers" had an incentive to lie about authorship of their wares, with probably a very small chance of being caught and suffering repercussions.
You can find contested works easily on Google by looking for "pseudo-[famous author]" . A few examples:
Nux, spuriously ascribed to Ovidius https://en.wikipedia.org/wiki/Nux_%28poem%29
appendix vergiliana, supposedly a collection of youth works by Vergilius. https://en.m.wikipedia.org/wiki/Appendix_Vergiliana