ArcGIS Online Named Users can launch a hosted service without checking how many credits it will consume. Sometimes, they are not aware that they don't have enough credits to perform the analysis (e.g. geocoding) and no alert is set to warn the users. As a result, ArcGIS Online will keep consuming credits even when none is available, resulting in negative credit balance. There should be a mechanism that limits the credit consumption to the number available in order to prevent this situation.