This is caused by a bug. This problem happens when a mac-address is authenticated and then is learned on another port that does not do netlogin. This could happen if for example the mac-authenticated used is learned on an uplink, for example when a mobile user roams to another AP connected to another switch. When this happen the mac-address is not removed from an internal table which could fill up over time.
The following log entries will show.
09/10/2016 18:16:53.54 <Info:nl.ClientFdbDel> : Network Login user 001122334455 cleared by FDB deletion, Mac 00:11:22:33:44:55 port 24 VLAN(s) "vlanclients"
09/10/2016 18:16:53.53 <Warn:HAL.FDB.MacVlanAddFail> : MAC-based VLAN entry 00:11:22:33:44:55 vlan 1426 addition to port 24 failed, Table full
09/10/2016 18:16:53.50 <Info:nl.ClientAuthenticated> : Network Login MAC user 001122334455 logged in MAC 00:11:22:33:44:55 port 24 VLAN(s) "vlanclients", authentication Locally