Pengi - 8:14 pm on Jan 24, 2007 (gmt 0)

Could the problem be something to do with you targeting a site with an ad that doesn't have space to display the ad.

Maybe there is a flaw in the Google Algorithm ("surely not!" I hear you cry) the registers an impression before realising that it is not possible to actually display the ad. This would seem to me to account for all the sympthoms you describe.