This morning I couldn't connect to one of my servers. I started to feel some panic, while I submitted a support ticket. Before I got a response, I figured out what the problem was.
On Friday, I tested XNmap by doing a port scan on this server. I forgot that I had installed portsentry, so the port scan added my ip address to iptables to so all connections are dropped, and to the hosts.deny file so services would reject these connections.
So my tips for the day are pay attention to what you are doing, and think about the unexpected side effects could be. Also, try not to run tests on production machines.