Should be fixed now, and thank you BurpZ.
I hadn't been throwing TribalFusion any ads for quite a while. Finally fired them back up last night.
When they didn't have an ad to serve, they were redirecting to an ASP on the old adserver which would fetch an ad from another agency.
So about 60,000 ad impressions got wasted. It's not so much the 50 cents that bugs me as the fact that people were seeing an error message where the ad should be.
I should point out, in case an advertiser is looking in, that the problem only affected delivery of agency ads. Ads from TheStreet.Com and CyberTrader were working fine, as were agency ads from TribalFusion. It's when they didn't have available inventory and tried to pass off to ValueClick that we were having problems.
The fix was to change the way the default was set up at TribalFusion so it'd get the ValueClick code from our *new* adserver rather than the old one, which no longer exists.