Appeared to be not documented at all what is not very good.
Suggest page https://www.zabbix.com/documentation/2.4/manual/config/items/itemtypes/zabbix_agent
In case of <mode> defined as "nowait" the key immediately returns just "1", disregarding of command execution results.
Zabbix agent 2.4.7rc1:
Also, another thing. Looking at https://www.zabbix.com/documentation/2.4/manual/appendix/command_execution and reading first sentence I feel that something is wrong.
Is that correct that "remote commands" are included to the list ?
As I know they are executed used "nowait" flag, so it's wrong to include them to the list, IMO.