I noticed that for several of my accounts, I had these mysterious impressions and clicks for the "unassigned" category (I have zero keywords in that category for all my accounts) on the day of April 13. The clicks and impressions show up in the category summary report, but NOT in the category detail report. But I HAVE been charged for them. I have not yet gotten a satisfactory response from Overture (or Yahoo Search Mktg) tech support.

Has anyone seen something similar on that date??

After three emails where they were trying to explain away the problem, they finally replied with the following:

"We apologize for the confusion. This is a known problem that we are currently working to resolve. We do not have an estimated time for repair however our technicians are working fast to resolve this problem."

So just be aware that your category reports could be way off.