As far as being full-blown 'fired', it was probably for arriving 5 (or fewer) minutes late to work three times within my first 3 months at a job. The rationale for that policy was that it was a store in a mall, and the mall's policy is that if you don't open by a certain time in the morning (I was an opener), then the store is fined $100 for each day that occurs. My being a few minutes late, of course, never resulted in us opening late - I would just work that much faster to make up the lost time, but that's how the policy was justified.
I was late because I had to use public transit (~1 hr. travel time+) to get to work. The first time I was late, it was because the bus came very late that day for some reason. The two other times I was late, it was because construction was going on along the bus route in front of the mall, and it slowed things to a crawl. I tried to leave earlier to compensate for such things but it still wasn't enough. The manager was very strict in following the letter of the policy and firing me, despite my actions not costing them anything. I feel as though she just didn't like me that much and was looking for an excuse.
Then there was a time I was almost fired from being a U.S. federal census enumerator (the guys who go door to door conducting the survey). The reason I was (nearly) fired was because there is a policy that says you must succeed in completing the surveys for so many residences per hour, and I was falling slightly below that. The reason is because you are assigned certain neighborhoods to visit, but often people are either not at home (there is a thin margin of time between when most people get home from work and when it would be inappropriate to knock on their doors, plus they are often eating dinner at some point in the meantime - and I worked in an area where people commute long hours, so they may not get home til early in the morning), or people will refuse to answer the door or otherwise take your survey because they detest the federal government and people who work for them (which is way more people than you might think). They sent a supervisor out to go around with me (who just made my job harder because he was an overbearing prick to me and the people I was trying to ask to take the survey), and together we still didn't have any better luck. So they withdrew their threats to fire me.
So yeah, basically was almost fired for sheer bad luck.
I also had an absurd lay-off situation, but I already talked about that over in the thread about quitting jobs, so I won't go into it here.