I had an accident on vacation once and was hospitalized. As a result, for a short period of time, whenever I would get drunk I would experience 'word salad' - some words I was trying to say would get changed into some other word when they came out of my mouth. But I had no idea it was happening - I heard myself saying one word, but my mouth emanated a completely different word. I only became aware of it when someone told me about it when I was sober. All my other faculties were totally fine, it was just the words got mixed up.

It hasn't reoccurred, but i'm now very aware that what I experience as reality is completely subjective to how my brain is functioning. Anything I experience right now could be a different version of reality. But at the same time, now that i'm aware it has happened, I can keep that in mind and consider it in the future. I have to believe that different forms of mental illness allow for the same kind of self-awareness, but it depends entirely on what parts of their brain are functioning (like if they can control their emotions and think clearly, which is hard enough for "sane" individuals already)