One of the most stressful parts of being employed is knowing that any slip-up can cost you your job. Show up late? Fired. Have low sales numbers? Fired. Get into a full out brawl with your co-workers? Fired.
You probably deserve to get fired for reasons listed above, but there are certainly some reasons that are completely without merit. For example, body odor. That seems like a ridiculous thing to get fired for, but I can almost guarantee you that it has happened.
Here's another example: Everything on this list.