another recent example of &quot;never trust the client&quot; is the amazon kindle unlimited debacle.<p>in case you missed it: the client syncs the last page read. so you can buy a book, jump to the last side and amazon marks it as read, without checking for intermediate states.<p>the problem is: amazon pays authors by &quot;pages read&quot;. so people publish fake books with 3000 pages, let sweat shops try the book for free and just jump from the first to the last page, sync and it registers with &quot;this customer read all 3k pages&quot;.<p>now create 20 fake author accounts and 30 fake reader accounts, publish 20 fake 3k books (one for each fake author account and they&#x27;re practically filled with 3k pages of randomly scraped web content) and you got 20<i>30</i>3000 = 1.800.000 pages read.<p>if you are an aspiring author and publish a novella  with, say, 50 pages and 10.000 readers devour every single page of it, you&#x27;ve got 500.000 pages read.<p>the faker can do this in a week and take about 4 times more than, even though it took you 2 months to write it. payouts are out of a pot shared between all authors. thus, authors lose in the short run (less money now), cheaters win big time (a lot money now), amazon doesn&#x27;t lose money (now) because the payout is the same.<p>in the long run it&#x27;s still problematic because small authors are unhappy once they realize what&#x27;s afoot (and earn 1k instead of 10k). big name authors don&#x27;t care as much if they earn 1m or 1.1m. customers aren&#x27;t likely to buy the fake books anyway (cheaters take them offline before the free trial period ends) so they&#x27;re not really affected.