“Accidentally” buffing Morgana in patch 10.22
At the end of October 2020, when patch 10.22 was updated, the Teamfight Tactics community was bewildered to find that Morgana’s damage suddenly became massive. Strangely, the patch notes for 10.22 on Riot’s website made no mention of Morgana. It wasn’t until Riot Mortdog, the lead designer of Teamfight Tactics, confirmed that they had “accidentally” buffed this champion that the community understood the issue.

This “accident” by Riot made the Dawnbringer composition “unstoppable” in patch 10.22. This team comp was already quite strong with Talon’s high physical damage. Morgana suddenly becoming a carry added a powerful source of magic damage to the Dawnbringer team. Therefore, it was very difficult for the Dawnbringer team to be countered, as it was rare for players to have both high armor and magic resistance. To rectify the situation, Riot reduced Morgana’s damage and heavily nerfed Talon shortly after.

Rework of the Mystic in patch 10.21
Perhaps due to the Mystic units being too weak prior to patch 10.21, Riot did not realize how unbalanced the additional true damage effect of this trait was. When they reworked the Mystic in patch 10.21, that effect immediately showcased its “game-breaking” potential with the Warwick “Raikou” composition. The principle behind this team comp is simple: equip Statikk Shiv to Warwick. The Mystic’s effect would amplify the true damage from this item to an extremely terrifying level.

Therefore, no matter how much armor/magic resistance the opponents had, they were defeated by the true damage completely bypassing those stats. This made the Mystic composition the most effective playstyle in patch 10.21. The meta in the first week of patch 10.21 likely revolved solely around who could get a 2-star Warwick first. After realizing the problem, Riot quickly nerfed both the Mystic trait and Statikk Shiv in patch 10.21b.
Buffing the Master system in patch 10.21
Patch 10.21 also witnessed another extremely unreasonable update from Riot, which was to increase the power of the Master system. This class was not weak at all, possessing high-quality champions like Shen, Yone, and Irelia with extremely powerful effects. However, Riot still decided to buff the attack speed slow duration of the Master system to 7 seconds at 4 champions (up to Elite). This made the Master system overly popular in patch 10.21 due to its significant capability to counter the meta.

Remember that the strongest compositions in Teamfight Tactics at that time were Duelist, Phantom – Spirit, which all revolved around physical carries. Even the “new” Mystic composition in patch 10.21 heavily relied on attack speed. By using the Master system, you only needed to use 3 spots in your team to counter most of the strong team comps in the meta. This led to the Master system being urgently nerfed just a few days after patch 10.21 was released.