I Can't Believe You've Done This refers to a viral video of an English teen starting a video, getting punched in the face, and replying, "Ah fuck, I can't believe you've done this!" The video was celebrated for the understated way with which the teen says the line, which people saw as a very "British" response to getting punched. Since being uploaded to YouTube in 2007, the video has been used as a reference in other memes, typically as a catchphrase, but also as a sound effect in lip-dub skits and other remixes.
So, basically, what I was thinking of was… Ah fuck, I can't believe you've done this!
Elsewhere online, the video was celebrated with a YTMND.  On January 11th, 2015, a thread about the video was posted to Reddit and gained over 4,100 points. The video has long inspired parody videos on YouTube. Over ten years since the original was posted, one of the most popular parodies, posted by Merlin Merlout on March 19th, 2018, features a cat and has gained over 2 million views (shown below, left). Another popular parody posted by kmlkmljkl used Peppa Pig and gained over 580,000 views (shown below, right).
On June 21st. 2021, PWee28 posted a video announcing a documentary about the video. The announcement video received over 352,000 views on YouTube in three months (shown below).
Temporary Removal From YouTube
On September 28th, 2021, Paul Weedon tweeted that YouTube took down the original upload of the video due to "violent content." The company later rejected the appeal submitted by Weedon in which he pointed out that the video was "a part of internet culture."
On September 29th, The Verge reported on the removal of the video, with Weedon providing additional comments in an interview with the publication. Users online also reacted with backlash after the video's removal, citing that it was a ludicrous decision. One such example was tweeted by Twitter user @tristandross, who said "this is like if someone at the louvre decided on a whim to throw the mona lisa in a skip."
Late in the day on September 29th, YouTube ultimately restored the video, stating that removing it was "a mistake on their end" in a tweet from their official Twitter account.