In the Star Trek: Prodigy season two episode, "Cracked Mirror," sections of Voyager-A are shifted into different universes (similar to the Star Trek: Voyager episode, "Shattered," in which different sections of the ship were shifted into different time periods).
The main cast from season one, plus Chakotay, are ascending toward the bridge and find themselves in a different universe on every deck they pass through. On one deck, they are in the mirror universe and Chakotay says:
The mirror universe. I know this one. Be careful.
I don't remember any episode of Star Trek: Voyager set in the mirror universe.
How does Chakotay know about the mirror universe?