Suppose we need to enable/disable an action by API.
And we want to use ONLY "actionid": parameter to select corresponding action.

Official documentation even has an examples with this operation, but it is partially wrong (actually disabled status is 1) and it does not work at all.
http://www.zabbix.com/documentation/1.8/api/action/update
http://www.zabbix.com/documentation/2.0/manual/appendix/api/action/update

Current trunk and 1.8 branch have different behavior.
In the trunk it works if we specify an action name (redundant data in our case)
1.8 branch does not work AT ALL.

See results of my tests:

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • Zabbix 1.9 latest (trunk) ******************************

>>>>>>>>>>>>>>>>>>> OK <<<<<<<<<<<<<<<<<:
request:
{
"jsonrpc":"2.0",
"method":"action.update",
"params":

{ "actionid":"17", "name":"test19", "status":"1" }

,
"auth":"ebe9ea3b39b7b7f00b39a04b50b3fdc8",
"id":2
}
response:
{
"jsonrpc":"2.0",
"result":

{ "actionids":[ "17" ] }

,
"id":2
}

>>>>>>>>>>>>>>> BAD <<<<<<<<<<<<<<<<
request:
{
"jsonrpc":"2.0",
"method":"action.update",
"params":

{ "actionid":"17", "status":"1" }

,
"auth":"356d584f180937a9eeab315ba2935056",
"id":2
}
response:
{
"jsonrpc":"2.0",
"error":

{ "code":-32602, "message":"Invalid params.", "data":"Action \"\" already exists." }

,
"id":2
}

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • • Zabbix 1.8 ******************************

>>>>>>>>>>>>>>>>> BAD <<<<<<<<<<<<<<<<<<<<<<<
request:
{
"jsonrpc":"2.0",
"method":"action.update",
"params":

{ "actionid":"3", "name":"18test", "status":"1" }

,
"auth":"bfcbc8dc41438a20652f4e0a672df8e5",
"id":2
}
response:
{
"jsonrpc":"2.0",
"error":

{ "code":-32602, "message":"Invalid params.", "data":"[ CAction::update ] Incorrect parameter used for Action [ 18test ]" }

,
"id":2
}

>>>>>>>>>>>>>>>>>>>>> still BAD <<<<<<<<<<<<<<<<<<<<<<
request:
{
"jsonrpc":"2.0",
"method":"action.update",
"params":

{ "actionid":"3", "status":"1" }

,
"auth":"1e9508bf3a409223cf9f0e0dae39a866",
"id":2
}
response:
{
"jsonrpc":"2.0",
"error":

{ "code":-32602, "message":"Invalid params.", "data":"[ CAction::update ] Action [ ] already exists" }

,
"id":2
}

>>>>>>>>>>>>>>> still BAD #2 <<<<<<<<<<<<<<<<<
request:
{
"jsonrpc":"2.0",
"method":"action.update",
"params":

{ "actionid":"3", "name":"18test" }

,
"auth":"7c6bd9ad0bf753a520e4f1c9a2258c15",
"id":2
}
response:
{
"jsonrpc":"2.0",
"error":

{ "code":-32602, "message":"Invalid params.", "data":"[ CAction::update ] Incorrect parameter used for Action [ 18test ]" }

,
"id":2
}