In many cases, it is OK to show slightly different content on different devices. For example, optimizing the smaller space of a smartphone screen can mean that some content, like images, will have to be modified. Or you might want to store your website’s menu in a navigation drawer (find documentation here) to make mobile browsing easier and more effective. When implemented properly, these user-centric modifications can be understood very well by Google.

The situation is similar when it comes to mobile-only redirect. Redirecting mobile users to improve their mobile experience (like redirecting mobile users from example.com/url1 to m.example.com/url1) is often beneficial to them. But redirecting mobile users sneakily to a different content is bad for user experience and is against Google’s webmaster guidelines.
A frustrating experience: The same URL shows up in search results pages on desktop and on mobile. When a user clicks on this result on their desktop computer, the URL opens normally. However, when clicking on the same result on a smartphone, a redirect happens and an unrelated URL loads.

Who implements these mobile-only sneaky redirects?

There are cases where webmasters knowingly decide to put into place redirection rules for their mobile users. This is typically a webmaster guidelines violation, and we do take manual action against it when it harms Google users’ experience (see last section of this article).   

But we’ve also observed situations where mobile-only sneaky redirects happen without site owners being aware of it: