I've found it just tends to have the primary connection die, and never tries to reconnect. ServerKeepAlives or what have you don't seem to help either - the link goes dead, and I won't be able to connect any other sessions because SSH will just keep on routing them into the control master.
I have had this problem, although fairly rarely. I have the following in my ~/.ssh/config:
ControlPath /tmp/ssh_mux_%h_%p_%r
This sets the path of the control file used to share the connection. If it ever hangs, I can just delete the file. But in practice I found this doesn't happen often and I appreciate the speed boost I get from connection sharing.
one minor annoyance is that there's a max limit to the ControlPath string (seemingly due to there being a max path length for Unix Sockets) which I've occasionally hit when connecting to hosts with very long hostnames (AWS default hostnames can sometimes hit it, IIRC).
Also note that the docs recommend against using publicly accessible dirs such as /tmp/ for storing your mux sockets. I'm not sure of the exact threat (maybe just info leakage about what hosts you're connected to, since the socket permissions themselves are strict), but I use ~/.ssh/mux/ for mine.
