Following Scenario:
1. I put some machines in Maintenance
2. The Status changes in "In Maintenance"
3. I delete the Maintenance (Configure, Maintenance, Delete Selected)
4. The hosts stays in Status "In Maintenance"
5. In Table hosts tha values from the deleted Maintenance are still there:
maintenanceid | maintenance_status | maintenance_from |
17 | 1 | 1318330840 |
6. In Table maintenance the maintenanceid 17 doesn't exist anymore

My suggestion, after Maintenance Deletion, all Values about that Maintenance should be deleted (put on 0) in the table hosts.