Can you trust your eyes and ears? Can we trust the eyes and ears of a character? Playing with narrator reliability can be fun, but it’s a double-edged sword.
Readers must never feel tricked or lied to, so substantiating the unreliability of a first-person narrator is important. “I’m a liar. I’m a known liar, that’s for sure.” Something like this tells readers right away that they probably can’t trust the narrator.
Another route—especially with a third person narrator is using the character’s own inabilities to skew perception. “The shadows at the edge of the yard played tricks on his eyes—he wasn’t sure if he saw movement.”
We’ve all experienced our ears and eyes playing tricks on us. It’s fun and useful to let our characters’ eyes and ears play tricks on them (and the reader) as well.