There are memory and calculation timeout limits for objects stored in the QVW files. And there are server limits for same items . As it turns out, it is a good idea to keep those limits to reasonable levels.
In some cases the QVS will use so much memory that it won't work anymore. In that case automate the process of measuring QVS memory + kill + restart. See here: http://community.qlik.com/forums/t/26373.aspx