Today, as I browsed Quora, I saw a question with an (apparently) blatant case error. I opened it, and curiously, one answer stated it could have been a correct case, since, while there is indeed "spiritus", fourth declension masculine, there is also a "spiritum", second declension neuter. Does such a word actually exist and, if so, does it mean "spirit" like "spiritus" or something else, and in the latter case, what does it mean?
Not in standard Classical Latin.
It's difficult to prove a negative, but Lewis and Short make no mention of it, and the Packhum corpus shows no evidence of forms like spīritī or spīritō.
L&S cite an inscription, Inscr. Orell. 3030, which has a dative spīritō (instead of spīrituī). In the comments, Alex B cites a dictionary of later Latin with an entry for spīritum, -ī. So this form definitely existed, even if Cicero wouldn't have used it.