There are definitely cases in which the possible values are ennumerable and known. I’m thinking more about situations in which a many-to-one relation, or other ambiguity is attempted to be collected. Such as coming across multiple values in a race/gender field in a police interaction database because multiple officers were involved. My problem with using “dirty” is that it is so ambiguous as to encompass relatively simple issues that can be translated via a lookup table, as well as messiness that actually reflects the messiness of the reality beyond the input form’s common case. In the latter case, there isn’t a “clean” situation as much as there is a rethinking of what you thought you were measuring.

Did the people reply with 50 really get the correct answer? Is this making the assumption that there is a "known" list of states, that addresses are entered correctly, that we're talking about a specific country?

The only way we really know the answer to this question is by placing a lot of bias into our answer.

50 states, probably. ;)

Edit: assuming a very US-centric dataset...