The default behavior: The player dies when he kills a teammate.
The same behavior occurs when -noTeamKills is enabled. (As expected, but counter-intuitive.)
It's also impossible to enable -tk when -noTeamKills is enabled.
While rare, team kills do happen from time to time when -noTeamKills is enabled. (Relates to custom plugins and rare edge cases.)
The impact of this unusual behavior could be reduced a bit with some changes.