I can't say there's ever been a "bad" update, as that implies I stopped enjoying the game.. but everything around Moon Lord while 1.3 was still being updated was completely hysterical. The arms race between players to beat this boss as easily as possible using existing game mechanics and the development team saying "no you can't use that mechanic" was absolutely legendary, and I think we've all forgotten it since we all joke about Torch Luck and the Torch God more.
The Moon Lord arms race is almost entirely responsible for the term "cheese" being used as a no-no word for somebody playing the game in a more advantageous way, or a way that's "less honorable" to you.
A timeline of the silliest update sequence known to man, beast, and god:
1.3.0.1 - June 30, 2015
Moon Lord is introduced. Everyone is just throwing Vampire Knives and Spectre Hood at it. Despite this being completely fair play for other encounters, and the massive DPS hit that these methods make you take to use them...
1.3.0.3 - July 2, 2015
Moon Lord now inflicts the Moon Bite debuff, which disables weapon-based HP recovery. Vampire Knives and Spectre Hood are completely killed by this update, since you otherwise will basically always have Moon Bite during this fight. Nurse and Heart Statues are still totally fine, though. Speaking of the Nurse...
Moon Lord did not yet have its teleportation behavior, so you could just kite it in one direction, Recall Potion back to a spawn point with the Nurse, and have all the time in the world to heal during the fight, even without auto-pause enabled. In response...
1.3.0.4 - July 9, 2015
Moon Lord will now automatically teleport to the player if too much distance is between them. This includes high-speed kiting, as well as teleportation. This sharply increased the difficulty of the fight, as the Nurse is no longer completely safe to use without autopause on.
1.3.0.5 - July 13, 2015
The Celestial Sigil now exists. The prior two weeks of this boss existing, it did not have a spawner, and you had to clear the Cultists and Pillars every. Single. Time. You wanted to battle the boss.. all while there were consistent updates that were effectively the rules of the game being changed on you in the silliest game of Calvinball I've personally been a part of.
Speaking of Calvinball, at this point, people started setting up invulnerability machines using the Slimy Saddle and Target Dummies. If you set things up just right, you would always have invulnerability frames due to always being in a target bouncing state, and you could trivialize every boss. Can you guess what happens next?
1.3.0.6 - July 20, 2015
The Moon Lord's attacks now have their own invulnerability timers, independent of the timers used by every other attack in the game.
This completely kills Slimy Saddle invulnerability machines. It also means that if a Solar Eclipse randomly occurs while you're fighting the Moon Lord, you're basically just dead, as you now have to deal with the Eclipse enemies firing projetiles at you from off-screen while you try to deal with the Moon Lord's attacks.
Note that since this was 1.3, our only infinite flight options were the UFO mount and the Cute Fishron mount. The UFO, compared to Witch's Broom and Soaring Insignia/Hoverboard (or Betsy's Wings, if you farm for that), is extremely mediocre - the handling is rough, and your hitbox becomes nearly twice as large. Cute Fishron suffers the same hitbox issues, and is also slower until you're at half HP, which is not exactly a state you want to be in when you're staring down the Moon Lord during a Solar Eclipse.
At this point, the Moon Lord is playing by its own rules, but these are honestly still fairly reasonable expectations. If these stipulations were in the game when the Moon Lord first released, there would have been complaining at the start, but people wouldn't have really minded. Unfortunately, the fact they were added slowly over time is just really bad optics. They go from the last boss being an otherworldly being that doesn't care about your mortal weapons to.. well, basically the game development version of a temper tantrum.
Fortunately, though, this was the last update to the Moon Lord that changed how you have to approach the boss. You have to build a proper arena to deal with staying at certain elevations for each of its eyes depending on class, and you may want to have a row of solid blocks to pull the Moon Lord above in order to deal with the Phantasmal Deathray, but it's otherwise
1.4.0.1 - May 16, 2020
The Phantasmal Deathray now goes through solid blocks between the Moon Lord and the player.
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