Can you use a flashback in a comic for something the person imagined happened? So basically in the flashback panels on my comics, we see the person having the flashback in the panel itself, which is impossible since he cannot see himself in third person and then the panel shows another person who was at a blind angle when he was experiencing the moment in first person, so is that ok, and is that even a flashback? The panels are in gray to tell the readers it's something that happened in the past.
Certainly you can. The important thing is that you need to clue the reader in that it's imaginary. Saving this for a reveal is dangerous because the reader may feel that the writer just threw that in to save the plot or something.
This requires either revealing to the reader that this character is not all there and is inaccurate in what he reveals (so that this is a part of a pattern) or specifically that this memory is invalid. Captioning, perhaps. Or having other characters recount something conflicting. Or perhaps putting literally impossible things into the flashback to warn.