Sic

[sic] is often mostly pointing out that this is not an error in transcription, it is intentionally reproduced exactly.

It's mostly used for literal quotation, pointing that we left unusual/archaric/bad grammar/speclling but we're leaving it as-is.

It is sometimes also used for things that are suprising for other reasons - suggeciently weird reasoning, or things that seem to come out of nowhere.

sic is apparently shorthand for sic erat scriptum, but almost no one would know that.