properly on IN lookups either
joins correctly but not the necessary cast if the target PK differs from the generic foreign key's fk_field (e.g. django comments uses a textfield) -- which results in a database error. The fallback methods handle proper casting.
across gfks -- this even extends to annotation and aggregation though the content-type must be filtered for explicitly.
sub-query of items
a lookup instead of both separately
items (i.e. today's most commented on blog entries)