On the section of marriageable girls in the Manusmriti, there are two verses:
For the Śūdra, the Śūdra girl. alone has been ordained to be the wife; for the Vaiśya, she as also the girl of his own caste; for the Kṣatriya, those two as also the girl of his own caste; and for the Brāhmaṇa those three as also the girl of his own caste—(13).
Under no circumstance whatsoever has a Śūdra wife been ordained for the Brāhmaṇa and the Kṣatriya,—even though these be placed in peril.—(14)
If the Manusmriti is the words of Manu, then why would Manu say marrying Shudra girls is allowed for Brahmanas, and then immediately after that say it "is ordained nowhere"? Manu himself ordained it in the previous verse!
One of the verses above is clearly an interpolation.
Commentaries don't help either. In fact, Medhatithi in his commentary admits that the two verses are irreconcilable contradictions:
Thus, then, since there is a clear incompatibility between the sanction (in 13) and the prohibition (in 14), the two should be regarded as optional alternatives.
This leads me to think that one of the verses is an interpolation. Some Brahmana did not like Brahmins marrying Shudra women, so he included verse 14; or in reality, Brahmanas are prohibited from marrying Shudra women, but a Brahmana included verse 13 out of desire of marrying a Shudra girl.