I sold online access to an event this weekend. And used ubercart to grant roles so people could access specific pages.
There were about 200 orders, and out of that 200 there were about 3-4 who in their order log it says ubercart granted role, but when you look at their account they didn't have the role granted under ubercart roles.
Here is a log from one that didn't work
04/29/2009
10:51:51 AM - Customer granted user role tryouts09.
04/29/2009
10:51:53 AM - Order created through website.
04/29/2009
10:51:53 AM - PayPal IPN reported a payment of xx.xx USD.
Here is a typical entry for one that did work
05/04/2009
3:46:13 AM - Order created through website.
05/04/2009
3:46:13 AM - Customer granted user role tryouts09.
05/04/2009
3:46:15 AM - PayPal IPN reported a payment of xx.xx USD.
The only thing that I can notice the in the one where the user didn't get access access was granted before the order was created. Not sure how else to help debug this.