Timeline
July 5 - September 22, 2016
Wild Pokemon Attack IV is bugged to correlate with Pokedex #. Pokemon with higher Pokedex # will have a higher Attack IV.
July 15, 2016
3-step tracking is effectively broken, never properly fixed, and later replaced with PokeStop tracking.
August 20, 2016
Pokemon who have moves that aren't attainable in the main series have their potential movesets fixed, creating the first wave of "legacy Pokemon."
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.
June 20, 2017
Last Premier Ball cannot catch a Raid Boss. The initial fix is to give players an extra Premier Ball.
July 17, 2017
Coin Exploit allowed bots to basically print coins. Niantic disabled gym defender bonuses in response until it was fixed.
July 24, 2017
Gym defenders return unfavorited, allowing trainers to accidentally transfer their favorited Pokemon.
July 28, 2017
Raid contribution bonus cleared when returning to lobby. Many raids failed because trainers who fainted would refuse to rejoin to finish off the boss.
January 20 - April 5, 2018
Game begins to remember which type of Poke Ball used to catch Pokemon across encounters. Unclear if this is a bug or intentional. Feature removed by April 5.
February 24, 2018
Dratini Community Day in Asia-Pacific region faces connectivity issues. Niantic extends event 3 hours.
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.
June 16, 2018
Larvitar Community Day in Asia-Pacific region faces connectivity issues. Niantic extends event 3 hours.
July 3, 2018
TMs lost in new item menu. Earlier related bug where TMing a Pokemon reorders where it appears (for example, 2 identical Golems), causing the trainer to TM the wrong one and waste TMs. Also the wrong move was listed.
July 21, 2018
Zapdos Day happens but does not go into effect for the Azores Islands, again. Niantic announces a re-do for the islands for August 3rd.
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 21, 2018
Beldum Community Day in Asia-Pacific region faces connectivity issues. Event rescheduled for the following weekend.
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 18, 2019
Several players report that Niantic is "losing accounts" which are unable to be recovered. A fix is announced about one week later.
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.
February 6, 2019
Species that shouldn't nest begin nesting briefly at the onset of the Kanto 2019 event, and are later removed and rotated into the raid boss set.
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 9, 2019
Lucky Friends feature is released, which guarantees Lucky Trades for Best Friends. Except sometimes it doesn't.
April 10, 2019
Legendary Lunch Hour blocks all of Mountain and Central Time Zones from receiving EX Raid passes, as predicted.
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.
June 4, 2019
Adding a 2nd charge move to legendary Pokemon briefly cost 25 candy instead of 100. This was reverted two hours later.
September 23, 2019
Ultra event Mewtwo caught after 4pm EST does not have Psystrike, leading to some frustrated last-minute raiders.
September 27, 2019
"World Tourism Day: Trade Pokemon caught 10,000 km apart" quest did not correctly invert Zangoose and Seviper regional rewards (new potential shinies) on some ocean islands and near some regional borders.
September 5, 2019 - Current
Players notice Paras has been missing from the spawn pool for over a month.
Note: This is arguably more "amusing" than "impactful".
