If during the intensive reading a log file is rotated, then item goes to unsupported state.

Example, we have a rapidly growing log file zabbix_agentd.log1, after executing commands that simulate rotation:
mv -f zabbix_agentd.log1 zabbix_agentd.log1.old; touch zabbix_agentd.log1
or
rm -f zabbix_agentd.log1; touch zabbix_agentd.log2
item "logrt["/var/log/zabbix/zabbix_agentd.log[1-9]$","error",,,skip]" with a high probability will go into "Not supported" state with error:
25754:20131106:133050.854 cannot process read entry '/var/log/zabbix/..'
25754:20131106:133050.855 cannot stat '/var/log/zabbix/zabbix_agentd.log1': [2] No such file or directory