The information the customer has written onto the form will be keyed into the computer, probably by a data entry clerk.  It will then be processed to produce a set of documents including a delivery note and invoice for the customer, as well as updating stock and sales records. 

There are several possible sources of error before  the data are processed:

The customer could make a mistake, entering the wrong product codes, adding up the total cost wrongly, forgetting to enter their address or card expiry date etc.

The person keying in the data could make a transcription error, keying in the wrong product code or quantity, misreading the customer's name, adding an extra couple of 0's to the total price by keeping a finger down too long, and so on.

A form could be blown into the bin by a sudden draught as a fan starts up or someone slams the door - or the operator might decide the writing was so bad it simply wasn't worth the effort of struggling with, and bin it.

A faulty connection between hardware components such as the processor and the disk drive could mean that some characters are wrongly transmitted.