If a hidden Twitter window decides to load and invisibly auto-play a video, your screen will never auto-lock. Proof:

% pmset -g assertions | grep PreventUserIdleDisplaySleep
  PreventUserIdleDisplaySleep 1
  pid 492(Twitter): [0x0002b542000525f7] 02:09:55 PreventUserIdleDisplaySleep named: "com.apple.avkit.disableUserIdleDisplaySleep"

But at least this idiotic slow-clap is in good company, since playing HDMI audio in iTunes does the same fucking thing.