Just did an apt-get update and upgrade on my ubuntu LEMP:

First got messages that it wanted to overwrite default and nginx.conf files, which I refused. Which resulted in

2012/03/15 20:36:05 [emerg] 19842#0: unknown "https" variable

Got this error previous time as well. Then I had to add the following to the nginx.conf:

map $scheme $server_https {
    default off;
    https on;
}

After which my server run happily ever after, until now.
This time I had to comment the https variable in the fastcig_params file to get past this. But now my nginx refuses to restart because of this:

2012/03/15 21:09:58 [emerg] 9599#0: bind() to [::]:80 failed (98: Address already in use)
2012/03/15 21:09:58 [emerg] 9599#0: bind() to [::]:80 failed (98: Address already in use)
2012/03/15 21:09:58 [emerg] 9599#0: bind() to [::]:80 failed (98: Address already in use)
2012/03/15 21:09:58 [emerg] 9599#0: bind() to [::]:80 failed (98: Address already in use)
2012/03/15 21:09:58 [emerg] 9599#0: bind() to [::]:80 failed (98: Address already in use)
2012/03/15 21:09:58 [emerg] 9599#0: still could not bind()

I don't have apache running. There are no nginx processes running. This is really annoying, second time that nginx breaks during upgrade.

The command netstat -ltnp returned following output:

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 127.0.0.1:3306          0.0.0.0:*               LISTEN      2181/mysqld     
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      2053/sshd       
tcp        0      0 0.0.0.0:25              0.0.0.0:*               LISTEN      2458/master     
tcp6       0      0 :::4949                 :::*                    LISTEN      2186/munin-node 
tcp6       0      0 :::22                   :::*                    LISTEN      2053/sshd     

What is going on?

In a final act of despair I removed all sites (sudo rm *) form the sites-enabled directory.
Then I made a link to my previous default vhost config. Problem persists.
Then I made a link to the new default vhost config (came with the upgrade). IT WORKZ!!!
Difference: in the new config file the line to listen for ipv6 on port 80 is commented out

#listen   [::]:80 default ipv6only=on; ## listen for ipv6

My server is ipv6 enabled, so is nginx, it worked before, now it doesn't. Anyway, I am bringing my sites online again by adding a # before the listen [::]:80 line in the vhost config files.. Very man. Problem solved, at least workaround. No clue as to why problem occurred, and how to make my sites ipv6 ready.

All the gory details are listed in http://wiki.nginx.org/HttpCoreModule#listen

I didn't really read through all of it, but having this line for only one vhost:

listen [::]:80 ipv6only=on default;

and only this for the other vhosts:

listen [::]:80;

seems to solve the problem.