Steps to reproduce:

Create UserParameter. Additional parameters passed via frontend:

UserParameter=custom.url.status[*],/etc/zabbix/userparameters/scripts/linux_url_status.sh $1 $2

linux_url_status.sh is a simple script, which returns status 0 if url is available or 1, if not. 
 

expected_output=$1
current_status=$(curl -sLk --max-time 10 $2 | tr '><[]' ' ' | egrep -i "$expected_output")if [ -n "$current_status" ];then
        echo "0"
else
        echo "1"
fi

 
Add some non working URLs

Result:
See screenshot...
Some of the items are returning 1, as expected, some - failing with Timeout error. In case of having a lot of such checks - other agent items may falling too. 

Timeout set to 20 seconds. --max-time for curl set to 10 seconds.