July 15, 2016
3-step tracking is effectively broken, never properly fixed, and later replaced with PokeStop tracking.
December 20, 2016 - April 30, 2019 (861 days)
Dodge glitch causes Pokemon to be switched out before they've fainted. Originally present in gym battles, a version of this bug manifest most impactfully in multiplayer raids, effectively discouraging dodging.
March 30-November 11, 2018
Research feature introduced. Trainers find they can "stack" Research Task Rewards almost infinitely. On August 7th Niantic announces a 100 encounter cap to be enforced on August 20th, however, Niantic actually enforced the cap on August 16th which was earlier than announced. As an immediate response to complaints, Niantic temporarily removed the cap on August 16th and announced a new enforcement date of August 27th. On October 4th, Niantic announces they will be restoring lost encounters, causing multiple bugs including multiple Mew and Celebi encounters, full stacks being lost, and stacks being recovered that didn't originally belong to the trainer. Restoration announced as complete on October 12th. Cap is announced again on November 8th, for implementation on November 11th.
July 28, 2018
The Pokemon Company announces an increase of the Pokemon storage cap to 2000, then recants the announcement. Niantic officially announces it, along with Gen 4, on October 16, 2018. Storage expansion doesn't release until November 21, 2018.
October 15, 2018
CP recalculation pushed. Most Pokemon gained some max HP, but Pokemon affected were not healed. CP recalculation is rolled back after some minutes, while Gen 4 Pokemon that release are calculated using the rebalanced formula.
October 23, 2018
Shadow Ball became TM-able on Mewtwo for ~20 minutes after Mewtwo leaves Tier 5 Raids, when the move is supposed to be legacy. Some players were reimbursed Raid Passes after sending in a support ticket.
November 1, 2018
Players who redeem their Research Breakthrough box immediately after 1:00 PM PT receive Moltres as a reward instead of Shedinja. A Raid Pass is given to players who send in a support ticket.
November 10, 2018
Cyndaquil Community Day in Asia-Pacific region faces connectivity issues. Niantic extends event 2 hours.
Metagross could learn Meteor Mash via evolution for ~45 minutes after Cyndaquil Community Day.
November 20, 2018
Trainers who linked their Let's Go Pikachu / Eevee games to their Pokemon GO accounts receive false ban warnings caused by a Niantic bug.
November 23-26, 2018
Umbreon and Espeon have near 0% catch rate when released into Tier 3 Raids.
November 26-28, 2018
Raid Bosses change 3 times in 3 days. Cresselia counters added to Raid Boss pool on the 28th.
December 13, 2018
Niantic releases Trainer Battles. Alolan Pokemon appear to have the effectiveness of their non-Alolan counterparts, but testing shows it's only a graphical glitch.
January 29, 2019
Palkia arrives as a Tier 5 Raid Boss, and Tiers 1 to 4 include a set of raid bosses that has been re-run several times before.
March 1, 2019
Dialga announced to replace Palkia as Raid Boss two days prior. Palkia rotates out February 28, 2019. On March 1st, instead of Dialga arriving, Palkia returns for over an hour.
April 29, 2019
The Lake Trio and Shellos are spotted for a limited time, and disappear.
May 19, 2019
Blaze Kick was incorrectly removed after Torchic Community Day ended at 7PM local time before finally being added to the regular movepool globally at 11:42AM EST on 5/19/19, causing some trainers to use charge TMs in vain, and other trainers to actually access the move before their local CD started.