Kids were warned not to cross their eyes, as one day they might get stuck permanently in that crossed position.
Your eye muscles can't stay crossed indefinitely, they'd fatigue and relax back to normal position within seconds or minutes. However, very rarely, extreme childhood strabismus (persistent eye misalignment) can develop from repeated eye strain, though this is not from casually crossing your eyes. Parents used this myth to discourage silly faces, not because of legitimate medical danger.
Reception
Sources
- Snopes: Crossed Eyes Stuck PRIMARY
- American Academy of Ophthalmology: Eye Myths REFERENCE
- Wikipedia: Strabismus REFERENCE