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:

{host: web.test.fail[Scenario].last(0)}

#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