In Fluent NHibernate 1.3, when trying to use OptimisticLock option in HasMany/HasManyToMany chain, it's not possible to set boolean value there, expected by NHibernate. Fluent NHibernate offers choosing one of options available only at entity level. Choosing any value results in XML validation error exception when creating session factory.
XML validation error
HasMany(x => x.Users).OptimisticLock.Version();
It should be:
HasMany(x => x.Users).OptimisticLock();
See also: http://notherdev.blogspot.com/2012/01/mapping-by-code-set-and-bag.html