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') 

 fixes it.

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.