On the Job

I've made a number of mistakes the past six months. I ordered books we already had copies of, paid a couple of invoices twice, lost my copy of the key to the building, forgot to consider the vendor discount in calculating our budget, and so on. Which of the following explanations is most likely? (1) Teaching philosophy for a number of years has made me stereotypically absent-minded, so the attention to detail necessary for acquisitions work will take me a long time to develop; (2) premature senility; (3) the menial aspects of librarianship are so boring that I do a slapdash job just to get them out of the way.