Can I sue a potential employer for refusing to hire me because of "Fluke" drug test?

Did you sign any type of contract accepting employment with this company? If not, there's nothing you can do. You're not officially hired until you sign a contract or show up for your first day of work, and the fact that this employer said they were going to hire you is not a legally enforceable promise. Your best bet is to wait it out. Assuming everything you've said here is 100% accurate, the chances are very good you'll be hired and you'll get an apology for the screw-up. Just remember if they decide not to hire you there's no point in suing them because you'd have no case.