Got burned today by YBYL. Totally my fault. I saw everyone had been cleared into first on the app and I was #1 out of 25 on the upgrade list following the clearing. So, I delayed my boarding until zone 3 started to get on which is when I approached and asked the GA if all of F had boarded; he confirmed they did. So, like an idiot, I just sauntered on the plane into my seat. I noticed when I boarded, however, there was one open seat in F. And we took off from ATL with that seat still empty. I just wasn't patient enough to wait for every soul to board; I will be next time.