When this occurs (also called an 'invalid redirect') the payment has failed and an email is sent to the merchant with the subject line: "Error in Form Submission".
The body of the message will resemble the example below with the highlighted items indicating variables for which incorrect values were sent:
An error page was displayed to the customer.
The following errors were found:
* x_line_item invalid
* x_amount : The payment amount (x_amount) is missing or not in the valid format.
* x_show_form : Parameter x_show_form must be PAYMENT_FORM
* x_fp_hash : Could not validate the integrity of the payment from the transaction key, x_fp_hash, x_fp_sequence, x_fp_timestamp, x_amount, and (optionally) x_currency_code values of the request.
* x_line_item # 1 - Price is not a number
* x_line_item # 1 - Quantity is not a number
The parameters of the request are
x_fp_timestamp : 1238797199
x_amount : abcdef
x_type : AUTH_CAPTURE
x_login : WSP-GOODS-70
x_fp_sequence : 123456
x_show_form : SHOW
x_fp_hash : bad
x_line_item : nice deal for one dollar