Just started happening: the AJAX response to a change in shipping method (or payment method) shows up as raw text on the screen:

{"status":"ok","errorMessage":"","orderTotalHtml":"\n Sub-Total:<\/td>\n $156.00<\/td> \n<\/tr>\n\n FedEx (Fedex 2 Day):<\/td>\n $37.82<\/td> \n<\/tr>\n\n 7 % FL Tax:<\/td>\n $10.92<\/td> \n<\/tr>\n\n Total:<\/td>\n $204.74<\/td> \n<\/tr>\n","shippingHtml":"","shippingMessage":"","paymentHtml":"\n
\n
Payment Method<\/h4>\n
\n
Please select a payment method for this order.<\/p>\n

\n
\"paypalwpp\"
\"Click <\/span><\/label>\n <\/div>\n
\n
\"paypaldp\"
Credit Card<\/label>\n <\/div>\n\n
\n First Name:<\/label>\n
\"Scott\"

Console shows error:
error: status=parsererror, errorThrown = SyntaxError: Unexpected non-whitespace character after JSON at position 5043 (line 1 column 5044), override: [object Object]