They could be using some node.js framework they don't understand that is silently persisting data.

Especially if they are using isomorphic JS. Globals / singletons are very dangerous when the JS runs on both the server and client.

* Pull data from API for pricing, later add caching

* Modify value to give local amount, not realising it's modifying a cached value

* Never test refreshing doesn't change the price

Caching.