er ... ok. so this seems to be widget logic related
in some cases, images enclosed in the tag do not link properly. instead of making the entire image a clickable unit, only the very top of the image can be clicked ..
http://www.thehealthcareblog.com - the 300 x 250 unit in the upper right corner
using widget logic to display sidebar content, including ad units
seems to happen unpredictably ... may have something to do with the coding on the actual widget (an outside developer modified our out of the box theme). note that oddly enough the second image down in the right hand column works properly ..