I have seen these in the event viewer on my DCs for a while and have never been able to figure out the cause. It has never caused any issues that I am aware of but I would like to see if i can figure out what exactly is going on to cause these messages. And hopefully resolve them.
Our domain is still 2003 forest and function level but we have performed the prep for 2008 r2.
Event ID 675
User Name: usrname
User ID: domainname\usrname
Service Name: krbtgt/domainname
Pre-Authentication Type: 0x0
Failure Code: 0x19
Client Address: 192.168.100.149
These will show up with different users from within our organization but as I said never seen to cause any issues such as lockouts etc.