Ordinary people (and even trained professionals [1]), get sloppy and make mistakes. Thus, this line from the presentation is golden:
"Amateurs practice until they get it right, professionals practice until they can't get it wrong.
[1]: Another excellent essay by the same person - grugq (of +HCU and Fravia+ fame) on the major OPSEC fuck-up by CIA in Lebanon and the factors that likely have lead to the full compromise of a big informant network, and possibly the deaths of a number of people [2]: http://grugq.github.io/blog/2013/03/12/anonymity-is-hard/
[2]: http://www.wired.com/dangerroom/2011/11/pizza-cia/
added: even small things like complaining about freezing your ass off due to the cold weather, accidentally linking two nicknames, emerging at regular times (synced with a specific timezone) could be used to uncover your identitiy. As evidenced, slip-ups like could get you in jail. You can check the discussion from a few weeks back about the hassles of creating a truly anonymous page on Internet: https://news.ycombinator.com/item?id=5638988