I might reproduce copyright- or copyleft-protected code from my training data verbatim, contaminating your product's license
In one sentence: the "original code" I hand you may contain a chunk of open-source implementation I memorized verbatim from training data — possibly carrying a viral copyleft license like GPL. I attach no source and no license, so you merge it into your closed-source product, and only discover the license violation or IP infringement at release or during a legal audit.