Some ArcIMS sites are not set up to stretch the returned image to fit the requested envelope by default. This results in a map with data layers that overlay well in the center of the map, but not towards the edges. This can be solved by adding "reaspect=false" to the request (by tacking it on to the connection string).

For example, if your mapfile is in a projection other than EPSG:4326, the following layer will not render correcly:

LAYER
      NAME "hillshade"
      TYPE RASTER
      STATUS OFF
      TRANSPARENCY 70
      CONNECTIONTYPE WMS
      CONNECTION "http://gisdata.usgs.net:80/servlet19/com.esri.wms.Esrimap/USGS_WMS_NED?"
      PROJECTION
              "init=epsg:4326"
      END
      METADATA
              "wms_srs" "EPSG:4326"
              "wms_title" "US_NED_Shaded_Relief"
              "wms_name" "US_NED_Shaded_Relief"
              "wms_server_version" "1.1.1"
              "wms_format" "image/png"
      END
END

Adding "reaspect=false" to the connection string solves the problem:

LAYER
      NAME "hillshade"
      TYPE RASTER
      STATUS OFF
      TRANSPARENCY 70
      CONNECTIONTYPE WMS
      CONNECTION "http://gisdata.usgs.net:80/servlet19/com.esri.wms.Esrimap/USGS_WMS_NED?reaspect=false"
      PROJECTION
              "init=epsg:4326"
      END
      METADATA
              "wms_srs" "EPSG:4326"
              "wms_title" "US_NED_Shaded_Relief"
              "wms_name" "US_NED_Shaded_Relief"
              "wms_server_version" "1.1.1"
              "wms_format" "image/png"
      END
END