I am not building anything as sensitive as a Financial App, but I do to need to get User data from a remote Instant Messaging system's database.

I built a set of testing accounts with known (to my test-package) passwords, and patterned user-data (for example, address1=user1-address1-xxxx, where xxxx is unique test-id (in reality the PID.time() at the start of the test sequence)). This is the only way that I could think of to

• Reliably test the interface in the face of a shifting API (there is only one of me writing the test harness; there are thirty or forty folks in Engineering fixing bugs and extending the feature set)
• Keep from leaking of my personal account and password data.

The method of generating patterned data turned out to have a side-benefit, I don't have to keep re-initializing the dummy accounts with a known data-state. I can just leave the account in the updated state since I know that my next test run will generate a different test-id by virtue of the included time-stamp. I use the fact that the test-id is in the data to validate the test results (is the time-stamp different or the same? Is this what I expect?).

A few years ago, I was working contract for one of the local banks in San Francisco (one with branches all over the state of California and elsewhere). There was a formal standard for building 'dummy' account-numbers, to we could 'test on the live system'.

We had someone run a test on the Check Ordering software and 'forget' to cancel the order to the printer (Rocky Mountain Banknote, as I recall). When the checks were delivered, Someone started using them to pay for purchases at stores in the Los Angeles air-basin. Since the account did have a positive balance (no surprise here), the checks all cleared.

The Bank didn't twig to the scam for over a month, until the quarterly account-reports came out and showed activity against a dummy account that didn't come from a test-id. They initially 'solved' the problem by sending instructions to all of their Printers to report 'suspicious' account numbers while they figured out a real solution. This generated a lot of false-positives....

My contract expired seven months later, and they were still arguing about the best way to prevent the scam from happening again.

UPDATE: added a paragraph tag after the list close; cleaned up the wording in the first sentence.