It's not unusual for Wizards of the Coast to ban a card for one reason or another. Some are too powerful, some break the game, and some are simply (in hindsight) racist.
But there was one card that broke the game so much that players themselves decided to ban it first, resulting in WotC needing to institute an emergency ban on the thing.
With that, we go back to 1999, a year where everyone thought Y2K was going to happen and dial-up internet was still a thing.
The Urza block was going strong, and in February of that year, Urza's Legacy came out. Going into UZL, Magic: The Gathering was just getting over "Combo Winter" when they had to ban a lot of cards due to too many overpowered cards and combos. Urza's Legacy was supposed to bring back some normalcy. And it was sort of doing that...until players started playing the card Memory Jar card.
"Tap: Sacrifice Memory Jar: Each player exiles all cards from their hand face down and draws seven cards. At the beginning of the next end step, each player discards their hand and returns to their hand each card they exiled this way."
Soooo...yeah. You can see how this would be a problem.
Players hated it because it just broke the game. Having a one-card nuke wasn't exactly what people wanted, so many players just stopped playing or started instituting house rules within tournaments to not play that card.
In context, it would be like the MLB players union coming together and saying that pitchers could no longer pitch sliders or if Nintendo players banded together and said no one could ever play as Wario in in any of their games ever again. No one wanted a card that made quick wins on the regular.
That's when WotC saw what was going on and introduced their first, and to date only, emergency ban on a card. It was really that bad. Very quickly, the DCI (at the time Magic: The Gathering's ruling body for sanctioned play) banned the card completely.
We'll let Magic's own Randy Buehler explain:
"The one card that was ever subject to an emergency ban was Memory Jar, which has the unfortunate text "draw seven cards" on it. However, the power of Memory Jar itself isn't why the DCI broke with its normal policy of quarterly changes. The only reason the DCI chose not to wait until the next regularly scheduled date was because the very health of the Magic game was being threatened by "Combo Winter."
Urza's Saga was four months old when Memory Jar came out in Urza's Legacy. During those four months, there was a large and loud public outcry about the way the game was being ruined by all the "broken" cards in Saga. [Players] either played against a steady stream of combo decks, or they didn't play at all. Players began leaving the game in droves. It was vitally important to the health of the game to clean things up before too many more players walked away, so quite a large number of cards were included in the DCI's March 1, 1999 announcement, which would become effective April 1 of that year.
Players were optimistic that Combo Winter was finally going to end. That's when Urza's Legacy came out and introduced yet another broken combo card to the environment. The stakes were high and the DCI did not want to see Memory Jar undo all the work they were trying to do that March, so they issued an emergency ban."