Not taking the time to apply for good internships during college.

I fell into a similar failure mode. My program requires internships, but I hate the job-search process, so when I got an offer from a good company, I accepted it pretty quickly, even though it was so early that many of the companies that I had applied to hadn't started their interviews yet. I forgot to ask myself which aspects of the job I wouldn't like, and was too impatient to stop searching.

(Disclaimer: if anyone happens to know where I work, it's really not a bad place. It's just not a good fit for me.)