One of my customer who has reported that on Exchange Server 2000 his Store event sink (onSyncSave) for global events causing the notifications for the couple of empty flags. When we analyzed his event sink, he registered it as,

regevent add onsyncsave sink.sink.1 "file://./backoficestorage/%userdnsdomain%/MBX/SystemMailbox{GUID}/StoreEvents/GlobalEvents/<reg>"

This issue happens whenever he created a new user and immediately he try to send the message in the user mailbox. When he tries to create OnSyncSave global event bindings in the Global Events folder for the store. For this they receive extra OnSyncSave folder event notifications, that are generated with empty bstrURLItem and EVT_ERROR | EVT_IS_COLLECTION bit set in lflags

Per the KB article this happens due to following: when a new user mailbox is created, the store automatically creates many subfolders such as Inbox, Outbox, and so on. The store delays such creation until the first item is created in the user's mailbox. Thus, when a new message is created in a new mailbox, extra OnSyncSave folder event notifications related to mailbox creation are generated.