I take this blog post as confirmation that:

1) ANY one message can be intercepted even if the sender exhibits ideal levels of alertness [Whatsapp server drops message to recipient; sends a rekey request with a fake key; message is intercepted since fake key was generated by server. Sender will see a warning if they turned on that setting (default is to show no warning), but it's too late].

2) Only Whatsapp has this vuln, not Signal app.

3) Depending on sloppiness of sender, more extensive interception is possible. [E.g., server not supplying delivery reports + sender doesn't have warning for key changes + sender sloppy about noticing lack of double check mark => full transcript can be generated]