> FOSDEM is free to attend. There is no registration. Just turn up!
I was quite surprised to see that. Even free to attend conferences usually require registration so that they can plan ahead and prevent overcrowding. Did FOSDEM not have this kind of problems in the past? Judging by the number of people I know that regularly attend it, it's quite a well known event.
If there is a talk that you don't want to miss, arrive before the previous talk ends, this way you'll be able to enter as soon as the previous talk ends. On really popular rooms it has happened to me in the past that no one got out and I was not able to attend the talk. Since (almost) all sessions are recorded and made available soon after FOSDEM ends, I watched it at home later.
They don’t do anything to prevent overcrowding, and so it is indeed painfully overcrowded and you may not be able to get in to see any talks you planned to. You basically have to pick one room, get there early, and then just camp out all day.
Each room is also not equal of size. The key note room is massive so there is always a seat there, but some is just sized for 30-40 people. When a famous project is talking in such a room it will fill up extremely fast, or in the worst case, be already filled up from the previous talk and people will just stay in their seats.
The common solution that other conferences do with this problem is giving out tickets to each talk and have people register exactly what they are going to see, but that in turn carries a equally massive issue in that there will always be empty seats from people simply not showing up. This in turn can lead to organizations trying to overbook in order to "fill the empty seats", turning into arguments and fighting when the calculated probability of an empty seat shows to be incorrect in reality.
I have been at FOSDEM for several years now and I never felt it to be overcrowded. Some talks will have a poorly chosen room (often based on topic genre than expected audience attendance), so a room designated for the IT security track can have 18 talks where the average number of listeners is 25 but then during a single session the Tor project has a talk in that track with an attractive title, and 100+ people are suddenly hoping to get into a room which is intended for 30.
Some things are busy, but there's usually plenty of others that are interesting or the chance to take a break and/or a beer and chill a bit.