Bugs should be submitted into Trac. You will first need to login using an OSGeo userid.
http://www.osgeo.org/osgeo_userid
http://trac.osgeo.org/mapserver/
Please keep the following issues in mind when submitting a bug:
The most important thing when reporting a bug is to boil down a minimum example that is needed to reproduce the bug. That means a minimal mapfile + any data files it depends on. Remove everything from the map file that isn’t needed to reproduce the bug.
The developers often dislike having to spend the first 30 minutes working on a bug, having to fix paths, remove unnecessary layers, removing references to external symbols or fonts that were not included or even needed and otherwise doctoring your test case to get it to a point when they can actually use it.
If the bug is easily demonstrated with “shp2img“, without the need to setup a proper web service and test it through http, then please show it that way. If a standalone MapScript script can demonstrate a problem without it having to be a web service, likewise submit it that way.
The chances of a bug being addressed in a timely manner is directly related to the speed with which the developer can reproduce the bug. If you make that hard for the developer, chances are the bug will be given up on or ignored for quite a while.