I'm sure that most readers at Quiet Speculation are familiar with MTG Stocks. If you headed over there today, you would see a substantial spike for Teferi, Temporal Archmage.

The card was about $10 and now there are only four listing on TCG Player, with the lowest listing being $16. I don't believe this card to have terribly high demand, and this looks to be a buyout that took advantage of low supply. If you look around you can find some $10-11 copies online, though it doesn't seem worth the effort.

The highest buylist price that I've seen is $5, which is half of the former market price. These aren't the easiest cards to move, and while the value has technically increased and the supply is likely to stay the same for some time, this just isn't a particularly sought after item.