Why redirect visitors
There are valid reasons for sending a visitor to a different URL than the one they initially requested, such as moving the site to a new address, or merging several pages into one.
What is meant by sneaky redirects
Sneaky redirects attempt to deceive search engines or display content to human visitors that is different than the content made available to crawlers. Doing so is a violation of the Google Webmaster Guidelines and may lead to your site being removed from Google and their partner sites. I’d strongly suggest that you never try to outsmart Google.
Types of sneaky redirects
There are many types of sneaky redirects, some examples include:
- Search engines are shown one type of content while visitors are redirected to something significantly different
- Desktop visitors land at the normal page, whereas mobile visitors are redirected to a completely different redirect
Where is the limit and how to avoid being penalized
It’s a tricky question that cannot be fully answered. What we can do is to look at what other websites are doing. Many newspapers, such as the Wall Street Journal, like to put a paywall in front of their content; so in order to be able to have all the content crawled and avoid being penalized, they choose to only trigger the paywall if the visitor is not coming from a Google search result.
There are also some image sharing websites, such as imgur that does not appear to be panalized for sometimes redirecting visitors from a direct image link to a page containing ads.
That being said, we cannot know for certain if this is acceptable behaviour, or if these websites are so big that different rules apply to them.