I've been fired twice:
1) A CNA didn't like me, so she smeared me. She told administration that I had grabbed her breasts (absolute lies, she would talk in great detail about her sex life regularly), and told all the African-American staff that I was racist (another nurse showed me racist text messages that the CNA had sent, on company time no less). I was fired as a diplomatic move, as there that CNA had ensured that there were people out for my blood. Everyone (maybe except administration) knew that she was a liar, a glass-house-dweller throwing stones, and a shrew, but like I said, it was diplomatic to fire me. That was about the same time they got new management and cleaned house. Many migrated to other jobs due to the new management anyway, so it was easier to explain to future jobs why I had gotten fired. By the way, the CNA later got fired for getting in a fight with another CNA, the classy broad.
2) The floor's nurse manager was fired, and the Ronald Reagan of nurse managers came in. Many of the people hired under the old manager were fired over trivial matters. One was fired for a one time no call no show which was probably due to the old manager's tendency to change the schedule or accidentally schedule one nurse for the entire floor. A CNA was fired, the reason they told her was because she didn't have a high school diploma. I was fired because my Pyxis record was audited after I admitted to losing a drug before I could waste it, and they found some incidents of me not giving the drug within 15 minutes after it was pulled (times got busy and emergencies happened, and the drugs were scanned in with bar code), and 2 isolated incidents of forgetting to waste excess. I don't pull without wasting anymore, and will sadly leave patients in pain because I'm not going to get caught up in JCAHO Witch Hunt.
Even the best of nurses will get fired. I know one who was fired because a unit secretary passive-aggressively refused to fill out or notify a certain nurse's orders. There will always be people who will not like you, there will always be stupid rules that go against humanity, and there will always be new regimes. Sadly, the biggest thing for getting fired is calling attention to yourself. It's easier to fire the one making waves than actually do something about the problem.