Photos I know I've taken are missing. There are periods of time when I walked around a city 10+ years ago taking a large volume of photos, like when I first moved to Seattle. Going back to that window of time in Google Photos, I only have a handful of photos from that walk.
There are also partially corrupted photos from many years back. Photos that only partially render, or render as noise.
Luckily, all of these are random low-value photos from my youth. They aren't core memories of my kiddos growing up or anything. I'm glad I discovered the data loss in that window of time, and not by losing photos of my kiddos' birthdays.