The standard YouTube video player reverts to the fallback (backup) player in instances of heavy traffic or network restriction issues. This is a way of supporting users with low bandwidth or in locations that heavily regulate YouTube content.
While the function of the fallback player is the same as the YouTube one, it does not have the same options as the full player, including not being able to change the video speed or personalize the appearance of the subtitles.
To return to the full Youtube video player, you will need to be on the lesson page. You can get there from any modal by clicking Go to lesson page.
Once you're on the lesson page, click the Switch to full player option at the bottom right of the video. This will reload the video in the full player, will remain in Khan Academy, and should return this player to your account's default player.
In addition, you can play the video directly on YouTube following similar steps.
As in the previous directions, select Go to lesson page while the modal is displayed,
Once you are on the lesson page, scroll to the very bottom (below the questions and comments) and click Video on YouTube.
Note: This approach will take you out of your Khan Academy account, and you will not get credit, will not be able to view progress, will not have an assignment marked as complete, and will not collect points or badges when watching the video on YouTube.