API token mismatch - do real client StoreId/API keys work in both prod and dev?

Question about the API keys - are the API keys provided by the clients for their production accounts also active in the development environment?

I've been using a store id and API key I generated to test on the development environment.  Now I'm putting the client's store id and API key in place, and getting an API token mismatch when trying to test against the development environment.  Will their credentials work against dev? Or only against production?

I would assume all credentials would be able to be validated somehow (such as against DEV) before putting them into production without positive confirmation they are working.