Most employees in the United States work without an employment contract outlining reasons why they can be fired. You probably are one of these folks. Unless you're covered by an employment contract or collective bargaining agreement, you likely don't have specific terms that outline your employment and are employed at will.
The concept of "employment at will" means that an employee can be fired for a good reason, bad reason, or no reason at all, including with no warning. This is what you seemed to face with your on-the-spot discharge for a mistake.
There are several important exceptions to at-will employment:
1) The key exception is employment contracts and collective bargaining agreements that outline terms and conditions of employment, including specific reasons why you can be discharged. In some states, employee handbooks/policy manuals or sections of employee handbooks/policy manuals are considered employment contracts. (Hint: Look to see if your employer has a progressive discipline policy in which the employer stipulates that it will discharge employees only for certain reasons, according to a given process.)
2) The reason for at-will job dismissal cannot violate state or federal law, such as whistleblower or non-discrimination laws.
3) Federal employees cannot be discharged for violations of the U.S. Constitution or the constitutions of the states in which they work.
Start by asking yourself if you have an employment contract or policy manual? If not and you aren't subject to one of the other exceptions, the answer is unfortunately yes, the office manager can most likely fire you on-the-spot like they did.
Sadly, this is one reason why people organize unions -- to do away with such arbitrary and capricious treatment. I'm sorry you received such poor treatment over a mistake. Don't make the assumption that you are ineligible for unemployment benefits. Here is an article that may help you move forward: http://hubpages.com/finding-job/12-Action-Steps-to...