Our dd11 had severe double vision that went undiagnosed for years because she never thought to mention it to anyone - she really truly thought we were all seeing two of everything.
Yeah, I used to get double-vision randomly, around ages 4-6, and I didn't think there was anything unusual about it. It was infrequent, and I regarded it as a curiosity. Something got picked up in an eye exam for my 4-years older brother, and the doctor said these things run in families, bring the rest in for testing, and that's how I was diagnosed. I had to wear bifocals for a year or so to fix it.
