Some common mistakes that will cause the designer to choke while persisting collection properties are:
The corresponding collection type cannot have multiple (overloaded) indexers. You should always define a single indexer of the type "public SomeType this[object obj]{...}". Otherwise the changes will be persisted but the control will not load in the designer. You will see a "Ambiguous match found" error. If the colleciton property has the PersistenceMode attribute set to InnerProperty then the property should not include a set method.