sorry if that has been posted before (couldn't find it on Google) :
GIF images generated with geoserver WMS are buggy when there is no feature in the image (when we zoom in to an empty spot for instance)
We're using a postgis repository as a source.
GIMP says they're incomplete, and diplays them with a broken line.
On the browser, they are displayed as partial, non-transparent layers, which makes them unusable for multiple layers display (like MapBuilder's).
If we go for PNG, then everything is fine, but we'd rather stick with GIF because of Internet Explorer not handling PNG transparency.
Have you encountered (and tackled) that problem before? Is it a JAI issue?