Have reviewed your install and went back 6 months worth of payments. Only this one has failed and did so in this way. It was a PDT, which means a redirect from PayPal back to your site. Based off the $transaction_info in the notification the API call to PayPal went haywire for whatever reason and PayPal returned an unexpected large chunk of HTML.
This seams like it was a temporary issue at PayPal. You've had several PDT and IPN notifications since then that were perfectly successful. Please let me know if and when this happens again as we might have to contact PayPal directly to see why their API endpoint failed like this. Specifically it was a call to their &cmd=_notify-synch that seamed to fail.