If JIRA has already reached the license limit , and then a new user is added from LDAP accidentally , JIRA will become unusable.

This error will be displayed on top of the page :

You have exceeded the number of users allowed to use JIRA under your license. You will not be able to create any more issues in JIRA until you reduce the number of users with the 'JIRA Users', 'JIRA Administrators' or 'JIRA System Administrators' global permissions. If you require more user accounts, please consider upgrading your license.

It would be nice to get a "grace period" for example make JIRA still usable when the license exceeds for a few days as upgrading the license with takes some time ( with an expert partner ).

There is also nothing stopping the users who are synchronized from LDAP to join the jira-users group even-though it has reached its limit.