Just out of interest, was you ad serving something from the insurance/health sector? Looking at G's CPC in adwords some of these areas are costing $40 per click.
Sometimes these types of things happens. Sometimes you earn big money from a single click whereas you earn very little money from a good number of clicks. There is no certainty in Adsense earning. Many times I have also got such types of giant clicks.
i've once got a click worth $9 for one click well this happened while i was editing my site but it never came back since it has been modified