I couldn't find an answer to this anywhere. Everything I find about overbookings is about people standing in line at the gate, after security scrutiny, and being bumped. I have read everything about it.
I will be flying soon next week within EU with a low cost airline that is not famous for being friendly to customers. I already know that my flight is full as no one can book additional tickets. Check in is not open yet, it will open 48 hours before flight. I could get a seat and a boarding card immediately for a fee if I wanted. I have no clue whether the flight is just full or overbooked.
Suppose, for some reason, that 1) the flight is already over-booked and 2) for reasons related to work and sleeping times everyone checks in and gets a free seat before me and my group
Here comes the question: according to EU rules about denied boarding and compensation, is there any difference between 1) being denied issuance of a boarding card with a seat number at online check-in phase, and 2) being denied boarding when you hold already a valid boarding card?
I think it is unlikely that the company could just say "oh, dear passenger, you were the last checking in, we have no more seats for you, forget about your trip and fly again with us". Really unlikely. I expect some form of compensation/arrangement.
Naive answer: one could always ping the arline website around check in time and be (among the) the first. It's unlikely that all passengers will be doing the same. But I am just asking a question to better know how it works.