In earlier versions of mapserver you might have a regular expression that looked like:
EXPRESSION ('[HAB_TYPE]' =~/^A1.1/)
If you try this with mapserver 6 however (especially on large datasets) you can get several mapserver threads starting - each of which ended up consuming 2GB RAM on the server - and never finishing
Changing the expression to:
EXPRESSION ('[HAB_TYPE]' ~ '^A1.1')
This is a nasty issue as most changes to the map file are "breaking" changes so it just doesn't work. This does run - but appears to kill the server.