Well, fun is the wrong word. Testing is rather a PITA, but it's one of those strange activities where you hope that it's going to be boring.
In this case, I discovered a bug which would affect hostnames that have numbers in them. I think that this came from a mixup in policy — originally I wasn't going to support hostnames with numbers but then I decided to — but anyway, it was screwing up the generation of archives (none of the beta accounts have numbers in them).
Well, all's well that ends well, especially when it's during the testing phase rather than the live phase. :-)
And, pleasingly, once again when I find a bug, it's not the sort that would cause a loss of data, just an inconvenience until it's fixed. I'm very pleased with the backend architecture so far. It's proved very resilient against data loss. In fact, [and I say this knocking on wood], I haven't seen a case of data loss yet as a result of a bug. Things are looking good.