> If you need to make a national payroll, you have to use it for a small town with a payroll of 50 people first, get the bugs worked out, then try it with a larger town, then a small city, then a large city, then a province, and then and only then are you ready to try it at a national level.

At a large box retail chain (15 states, ~300 stores) I worked on a project to replace the POS system.

The original plan had us getting everything working (Ha!) and then deploying it out to stores and then ending up with the two oddball "stores". The company cafeteria and surplus store were technically stores in that they had all the same setup and processes but were odd.

When the team that I was on was brought into this project, we flipped that around and first deployed to those two several months ahead of the schedule to deploy to the regular stores.

In particular, the surplus store had a few dozen transactions a day. If anything broke, you could do reconciliation by hand. The cafeteria had single register transaction volume that surpassed a surplus store on most any other day. Furthermore, all of its transactions were payroll deductions (swipe your badge rather than credit card or cash). This meant that if anything went wrong there we weren't in trouble with PCI and could debit and credit accounts.

Ultimately, we made our deadline to get things out to stores. We did have one nasty bug that showed up in late October (or was it early November?) with repackaging counts (if a box of 6 was $24 and if purchased as a single item it was $4.50 ... but if you bought 6 single items it was "repackaged" to cost $24 rather than $27) which interacted with a BOGO sale. That bug resulted in absurd receipts with sales and discounts (the receipt showed you spent $10,000 but were discounted $9,976 ... and then the GMs got alerts that the store was not able to make payroll because of a $9,976 discount ... one of the devs pulled an all nighter to fix that one and it got pushed to the stores ).

I shudder to think about what would have happened if we had tried to push the POS system out to customer facing stores where the performance issues in the cafeteria where worked out first or if we had to reconcile transactions to hunt down incorrect tax calculations.

You could have, in principle, implemented the new system to be able to run in "dummy mode" alongside the existing system at regular stores, so that you see that it produces the 'same' results in terms of what the existing system is able to provide.

Which is to say, there is more than one approach to gradual deployment.