See TODOs here (which I intend to remove soon after filing this ticket):
https://github.com/mongodb/mongo/blob/e768373f878c43100c023565c01ed16ad810ad1b/src/mongo/db/cursor_manager.cpp#L188-L189

https://github.com/mongodb/mongo/blob/e768373f878c43100c023565c01ed16ad810ad1b/src/mongo/db/cursor_manager.cpp#L198-L199

These are cases where someone attempted to kill a cursor but failed.