[Flenser]

It's a potential privacy violation for Alice and Bob!

How about this scenario:

Alice has Chrome synced to her Google account on her PC.

Bob uses Chrome on his PC but has no Google account and does not log in to Google services. He does uses bookmarks though.

Alice visits Bob and borrows his PC to check her gmail, which logs her in to Bobs Chrome. Then either Alice at that time or Bob at a later date accidentally triggers sync in Bobs Chrome.

TWO bad things happen at this point.

1) all Alice's synced data is downloaded onto Bob's PC. Including her bookmarks and passwords

2) all Bob's bookmarks are synced with Alice's account and Chrome on her PC will download them next time it's online.