The TCP/IP stack is compiled for 64 sockets. If the socket table is full, it just drops all new SYN packets and does not answer to them.

Just curious and totally not being critical, what was your thinking about dropping SYN vs responding with RST?

I think the user will most probably keep pressing the F5/Refresh button anyway if the site is not working at the first try, so responding with a RST packet would just use more bandwidth and resources.

iptraf on another computer, connected to the same hub: https://www.youtube.com/watch?v=0CeK1TnGhIs

Assuming that one "TCP entry" means one socket, that's a lot of SYN packets...

Video from what it looks like on the server: https://www.youtube.com/watch?v=BVKjVyM53sg