BTW, I just ran into it again and this time I decided to check the source code to see what the problem might be. It appears that the ad does not have a closing [/A] tag:

[A HREF="http://ad.doubleclick.net/jump/N1951.renderosity.com/B1090796;sz=468x60;ord=[timestamp]?"]
[TABLE border="0" width="100%" align="center" cellspacing="0" cellpadding="2"]

---

This is sample code from a working banner ad:

[A target="bannerad" HREF="http://www.renderosity.com/adclk.ez ad_no=1331&X=22137"]
[IMG SRC="http://market.renderosity.com/photos/AdExclFyrene.jpg" border="0"][/A]
[TABLE border="0" width="100%" align="center" cellspacing="0" cellpadding="2"]

Crooked brackets changed to square brackets so as not to be interpreted by the browser.