If you applied the October 2013 update for the Lync 2013 client, I'm sure you noticed that calendar-based presence updates broke. This was a huge headache for me, as I typically block off my calendar with an appointment for the customer I am working with. Normally, this sets my presence status in Lync to "Busy" so I don't get interrupted (ha!). With the October 2013 update applied, I had to remember to do this manually - which I often forgot to do.