There's no warning in the UI when memcached can't set maxconns because of system resource limits. There is a warning in the memcached.log.2.txt file, but no one is going to look there probably.
Reproduce is easy. Edit /etc/init.d/couchbase-server and comment out "ulimit -n 10240". Ensure hard nofiles limit is not set in /etc/security/limits.conf. Run "/etc/init.d/couchbase-server restart".
Look in most recent memcached*log*txt file in /opt/couchbase/var/lib/couchbase/logs, and notice this warning:
Thu Jan 17 14:08:51.673016 PST 3: WARNING: maxconns cannot be set to (10000) connections due to system
resouce restrictions. Increase the number of file descriptors allowed
to the memcached user process or start memcached as root (remember
to use the -u parameter).
The maximum number of connections is set to 1006.
Look in UI, at logs tab, etc., and notice no warnings anywhere.
There may be other conditions that should bubble up to the UI as well, but this is one I have noticed. This doesn't happen often, but some settings (maybe SELinux or something?) may prevent the ulimit from being run or taking effect, and it is important for the admin to know this.