We try to create schedule with outlook client. If any participants have a schedule conflict, following error appears.
"Scheduling error The Following potential problems have been found for this activity. Do you want to ignore these errors and save"

But following document said the message is a warning.

http://www.microsoft.com/en-us/dynamics/crm-customer-center/create-or-edit-an-appointment.aspx
If any participants have a schedule conflict, a warning appears. You can choose Ignore and Save to save the appointment anyway.

It is nice to improve the message as a warning.