Some won't let you check-in less than 45 minutes before your flight (or 60 for international). Only time I've missed a flight was because of that

Completely dead airport, got there 45 minutes before my flight, ended up missing the 45 minute check-in window by a couple minutes

They changed me to the next flight for free, and I just ended up having to wait around for a couple hours

This is out of thousands of flights the only time I've missed one. I usually plan to get there 60 minutes before my flight