Okay, here's a new one that YT has actually admitted to (yes, for once, they actually admitted that they created this problem): so, whenever you go to add a video to your favorites, then you go to your favorites, it looks like it isn't there right? That's because now, for whatever reason, whenever you add a video to your favorites, it goes to the very bottom of the playlist... so like, you have to search all the way back to the first videos you ever favorited, then you'll find the vids you recently favorited before those.
But, in order to avoid this, whenever you go to favorite a vid, you have actually check a box that says, "Add to top of list" to keep your favorites in the order of which you favorited them.
And again, for once, YT has admitted to causing this screw up... they didn't say why they did it, but for once, they at least said they did it.