If the visitor can not open the ticket, or scanning does not work, the organizer can always allow entrance through the dashboard. Or if you, by accident, scanned the wrong ticket, you can undo this through the dashboard too.
Look up the customer in the dashboard and select it. The visitor’s data will be showed. (Read here how to look up the customer)
Check-in
Select the tickets you want to check in and click on 'Batch action' and then choose 'Check-in'. The tickets are now scannend. 
Check out
If you have accidentally checked in a wrong ticket, you can always undo this via the dashboard. Select the tickets you want to check out, click on 'Batch action' and then on 'Undo check in'.