Take the item cache, for instance. If between two cache synchronizations a user removes an item and then creates another one with the same key on the same host, the item will disappear from item_hk index.

Similarly with hosts. If between two cache synchronizations a user removes a host and then creates another one with the same name, monitored by the same proxy, the host will disappear from hosts_ph index.

In trunk, the same problem can happen with interfaces, macros, and probably something else.