Zabbix server has the bug to send alert in maintenance period.
This phenoma occurs in following situation.
1. An alert is created with recovery message.
2. A triggper is on.
3. enter maintenance period
4. The trigger is off.
5. exit maintenance period
6. enter 2nd maintenance period
7. A trigger is on and alert is sent.

The source is that after maintenance period unneeded escation is inserted.

Same bug in 2.0.x is fixed in ZBX-6609. But in 1.8.x not fixed.