Significant enough that if the license is GPL (which some has been) it will "taint" the entire codebase and license it under GPL. Significant enough to be found by automated OSS audit tools, which would trigger a re-write and education for the developer who committed it.

EDIT:

> If I write down lyrics I remember I still wrote it.

Not from a copyright point of view. The rights to those lyrics belong to the songwriter. It's kinda like photographs. You don't automatically have the right to distribute a photograph of yourself that was taken by someone else.