If a host is removed (using e.g. configuration --> hosts --> hostname --> delete), then Zabbix keeps information about it somewhere in the database. Even the housekeeper process doesn't clean this up, as it seems. When a new host with the same name xor the same IP address is introduced later, then this leads to inconsistencies in the database (AFAICT). For example, I got tons of messages in zabbix-server.log complaining about unknown hosts "localhost.localdomain" or other host names that are currently not in use.

A partial workarund is to flush the dhosts and dservices tables. This is highly painful.