Famous YouTube channel Smosh released a 30-second promo advertising the channel around 2013-14 titled "What Smosh is in 30 Seconds". It shows various clips of various Smosh videos while intertitles appear describing the channel.
This ad appeared on YouTube as an unlisted preroll ad and has been very popular.
This video hasn't resurfaced anywhere on the internet since then.
Here's the link to the video, URL found by Windowsstartupsound: