On the web manual (http://www.zabbix.com/documentation/1.8/manual/web_monitoring/web_scenario), Section 9.3 WEB Scenario has the following text.
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Example 1
Trigger "WEB scenario failed"
The trigger expression can be defined as:
{host: web.test.fail[Scenario]}.last(0)#0
Do not forget to replace the Scenario with real name of your scenario.
Example 2
Trigger "WEB application is slow"
The trigger expression can be defined as:
{host: web.test.in[Scenario,,bps]}.last(0)<10000
Do not forget to replace the Scenario with real name of your scenario.
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The expressions should be:
#0
{host: web.test.in[Scenario,,bps].last(0)}<10000
At least, when I configured my server, it would not accept the existing examples, and the corrected examples are working fine 