Over the last few months, there have been some companies experiencing issues with Paypal IPN. The issue seems to be isolated to individual accounts and not an issue that is effecting all accounts.
The problem is simply this, when payments are made by customers, paypal does not send out the standard IPN. As such, the companies system will show the customers purchase as unpaid. This recently happened to StatCounter.com.
As it would seem that paypal have not yet addressed this issue, it is almost impossible to tell what the exact issue is or when or if it will strike. While StatCounter.com experienced this issue with all of their customers purchases, it may well be possible that some paypal accounts will only see this happen to a few customers. In the past, the paypal IPN system has gone down for periods of time, or errors were given during processing that should not have happened. As such, it is possible that some accounts may experience some failing IPN communications, or incorrectly formatted IPN callbacks. Until this issue is addressed by Paypal, there is nothing that can be done by those affected except to use another payment processor such as Google Checkout or Authorize.net to name a few.