I handle 10-20 payments daily from paypal and I use IPN to automate things.
20-30 minutes ago a payment went through.
I received a payment from subscription of £11~.
I received the subscr_signup IPN.
Then 10 minutes after or so I received the subscr_cancelled.
I checked IPN history and Paypal never sent out the subscr_payment that it sends out on all other payments.
I did receive the money and it was successful but Paypal never sent it out.
This gave me some mild headaches but I was able to add the records manually to the database. But it's a pretty bad experience. What if I was dealing with hundreds of even thousands of transactions day. No chance that I could deal with this happening often.