The 2017 February Sub Glitch was a Sub Glitch/Purge potentialy caused by Subscribes and Unsubscribes counting as -2, which happened on a collab between Ethan Klein and PewDiePie.

The Collab Video

The collab video was posted on YouTube in 8/2/2017. In which they reacted in sequence to:

After the first video, PewDiePie is ordered to "talk more", making the reactions more forced, and the videos reacted to get darker and darker, starting with a dumb musical video, then a pee fetish video, and then complete homosexual porn, with Ethan describing the video in full detail, liking it, calling it "fun" and making PewDiePie endure it.

The Glitch Video

The glitch video was posted on YouTube also in 8/2/2017. In which Ethan explains the Sub Glitch. With multiple Channels like: BuzzFeedVideo, LeafyIsHere, Tana Mongeau and DramaAlert losing thousands of subs. 23 minutes later. Leafy's the second to make a video about it, then PewDiePie a day later.

YouTube later explained[1][2][3] that the Sub Glitch was completely visual, not actually unsubbing to YouTubers 2 times[4].