In mapserver 8.4 (also happens in previous version) if you set your map projection to 3857 and configure a scalebar, teh resulting scalebar measurements are incorrect
`PROJECTION
"init=epsg:3857"
END # PROJECTION
LEGEND
KEYSIZE 20 10
KEYSPACING 5 5
LABEL
SIZE 10
OFFSET 0 0
SHADOWSIZE 1 1
END # LABEL
STATUS OFF
END # LEGEND

SCALEBAR
INTERVALS 4
LABEL
SIZE 10
OFFSET 0 0
POSITION CR
SHADOWSIZE 1 1
END # LABEL
POSITION LL
SIZE 400 6
STATUS EMBED
STYLE 1
UNITS METERS
END # SCALEBAR`

see outcome, data is in 20350 (projected) and label with distance is calculated using the coordinates in 20350 as a sperate (pre-processed) layer, these disctances are accurate. Scalebar generated by mapserver is not correct.