I usually allow 1 hour when I've checked-in online, and two hours where I have to check-in at the airport.
I'm yet to miss a flight (well, except by sleeping in by five hours after a really heavy night out but that was my own fault!). However, I came close at Stockholm Skavsta airport last year. Most people probably take the direct coach to / from the centre of Stockholm, but me being me, I chose to use the train via Nykoping. It was a holiday period and Swedish Railways were running a reduced service, although this wasn't a problem as I found trains which tied up nicely. On the way back, the ticket vending machine wouldn't give me a seat reservation, which I initially found a bit odd, but then I forgot about it and popped off to a pub for a final pint. Returning to the station 10 minutes before departure, I found the train, and squeezed on. It was PACKED... all the seats taken, and hundreds of standees, but no different really to trains here in the peak. However, departure time came and went, and I started to worry when we still hadn't moved 20 minutes later. Eventually, there was an announcement (in Swedish) and all standing passengers started to grumble, swear and move. I asked someone, and they said the train crew had refused to take the train out as it was too busy. Therefore, everyone without a seat had to get off and wait for the next train, two hours later! ARGH!
To cut a long story short, a lot of people refused to get off, and eventually they were "encouraged" to do so by the local police. The train was being prepared for departure 45 minutes late (already tight for my flight) and I was begging the conductor to let me on, else I'd be stranded. At the last second, after he blew the whistle, he opened the back cab door and pretty much pushed me in, climbed in himself, and off we went. I was the only person who had not originally had a seat to travel, and to cap it all had a great view out the back cab window and spent the journey chatting to the conductor.
Made it to Skavsta 25 mins before departure, and was last on the plane
