I think it's decently plausible if your Alexa hears you, but you don't hear your Alexa. So you continue on your conversation, and you say enough to keep progressing the flow.

I would assume any failed speech recognition attpempts are recorded so Amazon can have a human look at and classify them, but at the very least they probably keep logs, so they should have this information when debugging. Maybe we'll get a more detailed postmortem later.