Today I encountered something new in the logon event- I thought that was old hat and I knew all there was to know about that but I guess I was wrong.
The logon event (528/540 prior to Windows Vista, 4624 in Vista and Windows Server 2008) has a field called a Logon Type. This is a code that is passed into the logon API that tells the authentication system in Windows which policy to check the logon against. Windows has separate policy checks for network logons, interactive logons, etc., so that you can allow users to access a system in some ways but not in others.
The logon type code is, in C/C++ parlance, an enumerated value- it's an ordered list of numeric values, each with an associated name, and these are defined in a publicly available file in the source code (ntsecapi.h). In the source code, the values are always referenced by name.
Today on one of the internal aliases someone actually found a logon event with a logon type of 0- I have never personally seen one of these before and 0 is not defined in the SECURITY_LOGON_TYPE enumeration, so I would have assumed that it was a bug- but it turns out that we are aware of this case and use it occasionally for system logons.
So there you are.