Do you or would you consider firing this employee?

A second chance is in order, but with additional supervision and/or lower priority cases, until she's ready to do the job on her own correctly. Some people when they screw up, don't know how to just say oops my bad, and go for outlandish reactions like I was sabotage, it was a conspiracy, without thinking, often it's due to their inability to handle the stress of thinking they are in big trouble ( a somewhat knee jerk reaction) .

And ironically it's often someone in a higher position then them ( their boss/supervisor) who really must take the blame, as either they weren't trained right, or decent systems weren't put into place to prevent or at least catch mistakes before they became serious.

No, i would give a verbal warning, and advise of the correct procedure with an explanation of why the procedure is done. Perhaps your staff member was unsure of what to do, and didn't know who to ask for help -- people learn from their mistakes, and i doubt your employee will make the same mistake again

If not firing at least a written warning so if it were to happen again you have proof on paper to follow through with further discipline, and be sure she reads the write up and signs it.