In early 2005, a growing alarm grew in the SEO community, regarding the malicious use of HTTP 302 redirects. Normally used for benign purposes, malicious parties started using these tricky redirects to take advantage of an apparent bug in Google and 'Steal' the Page Rank of a target site.
Page hijacking is a form of spamming the index of a search engine (spamdexing). It is achieved by creating a rogue copy of a popular website which shows contents similar to the original one to the web crawler, but redirects web surfers to some unrelated or malicious websites. Not every site that uses redirects is trying to hijack your site. Page hijacking redirects have existed throughout the life of the internet without any problems. Webmasters that do redirects never needed to know what a 302 or 302 redirect was, they just used there preferred scripting language to redirect to another page for whatever reason they saw fit. It’s only recently that redirects have started to have a detrimental affect on the target URL as far as Google and some other search engines are concerned. When you do a inurl:www.example.com query on Google (to check for hijacking) not every website listed is actually hijacking your site. The way to tell if your site has been hijacked (intentionally or not) is do a inurl:www.example.com query on Google then any of the sites listed with exactly the same title, snippet and cache as your webpage/website are actually hijacking your site. If you click on the cache link next to the culprit you will see an exact copy of your site that resides on someone else's URL. That is hijacking (content theft). The 302 redirect problem all boils down to duplicate content. When Google has the same title, snippet and cache (identical copy of your web page) for your web page and other pages in its index this trips a duplicate content filter. It's Google's choice which page it gives credit to for that content. It is then only a matter of time before all other pages with that exact same content are shifted to the supplemental index.
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