Today we received this message when attempting to create issues:

When our administrator looked at the number of user accounts, there were 101 (we have 100 licenses).

Allowing customers to create more users than they have licenses for but bringing all existing users to their knees when they do is a terrible business model. JIRA should prevent creating the extra user in the first place and notify the administrator at that time that the license limit has been reached. This would guarantee that the administrator would be the first person to see the error and allow them to resolve it while allowing existing licensed users to continue being productive.