It's actually not very hard to cause. It would happen if your system reuses buffers without actually going through the allocator. Something like this:

    let mut buf = vec![0; DEFAULT_BUFFER_SIZE];
    let database_frame_len = database_socket.read(&mut buf)?; // use 1
    let database_result = Database::parse(&buf[..database_frame_len])?;
    let html_template = HtmlTemplate::new(database_result);
    let html_len = html_template.write(&mut buf)?; // use 2
    socket.write(&buf[..html_len]);

The above code makes the tenuous assumption that HtmlTemplate::write actually clobbers everything it's supposed to write. But it doesn't even require unsafe code for it to not do that, because the underlying buffer is entirely initialized according to the type system.

The only advice I can really give to avoid this kind of bug is: don't reuse buffers unless it's actually a bottleneck.