tl;dr: Your government can install spyware on your PC (enabling surveillance that would normally be prevented by standard security features) when you go to download legitimate software from a real website, without requiring any error on your part; in some places this is happening

Edward Snowden on Twitter

“Huge: @Citizenlab catches ISPs invisibly redirecting download requests for popular programs, injecting them with government spyware. Unencrypted web traffic is now provably a critical, in-the-wild vulnerability. 20-30% of top internet sites affected. https://t.co/5RR8BlkicH”

^twitter.com