another recent example of "never trust the client" is the amazon kindle unlimited debacle. in case you missed it: the client syncs the last page read. so you c

another recent example of "never trust the client" is the amazon kindle unlimited debacle. 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. the problem is: amazon pays authors by "pages read". 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 "this customer read all 3k pages". now create 20 fake author accounts and 30 fake reader accounts, publish 20 fake 3k books (one for each fake author account and they're practically filled with 3k pages of randomly scraped web content) and you got 20303000 = 1.800.000 pages read. 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've got 500.000 pages read. 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't lose money (now) because the payout is the same. in the long run it's still problematic because small authors are unhappy once they realize what's afoot (and earn 1k instead of 10k). big name authors don't care as much if they earn 1m or 1.1m. customers aren't likely to buy the fake books anyway (cheaters take them offline before the free trial period ends) so they're not really affected.