Since it is always better to learn from the mistakes of others than to fall down yourself, avoiding what Google has shared would help your smartphone users engage with your site even more fully, while it also helps searchers locate what they are looking for with added speed and accuracy. Some of the more common mistakes include faulty redirects and of course, smartphone-only errors such as when you recognize a user who is visiting a desktop page from a mobile device, and you happen to already have an equivalent smartphone-friendly page at a different URL, it would help to just redirect them to that URL other than to serve up one of those 404 or a soft 404 pages. Good luck in making your mobile page experiences better for your visitors!