Why does Rails limit 1 when calling Post.find? Shouldn't this query only ever return 1 record?

Say when finding an id, this can either be a specific id (1), a list of ids (1, 5, 6), or an array of ids ([5, 6, 10]). In the instance of a list or array of ids, I'm guessing the result will not be limited to one

That is the expectation when you use find with one argument. The Limit clause is probably there to ensure that the expectation can be relied on regardless of the state of the database. Sometimes it's ok to be extra explicit even though it might not be necessary.

I imagine it was a conscious choice also to be consistent in the implementation of any method that the client expects only one item returned.