Yeah, those hidden sprinkler lines will get you every time. I've seen guys on sites hit one and suddenly there's a geyser shooting three feet in the air, soaking everything. It makes a huge mess and can wreck the pressure for the whole system. Always, always call 811 or use a locator service first, it's free. That five minute call saves you from a very bad, very wet day.
On a project in Austin last year, a guy hit a sprinkler line and flooded the whole basement. Water was everywhere for hours, ruining drywall and flooring. Joseph is right, calling 811 is a no-brainer. Skipping it is pure laziness and can cost thousands in repairs.