These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Acceptable Inevitable Targets: Elderly women, portrayed here as stereotypical cookie bakers that get upgrades like better dentures and prune juice, and eventually turn into wrinkly, tyrannical, cosmic Humanoid Abominations that take over the planet, become the embodiment of mass cookie production, and play bingo.
Fridge Logic: overlaps with Fridge Brilliance and Fridge Horror - Players who develop enough cookies-per-second are informed that their universe is now made of cookies. Portal Devices import cookies from alternate-universe-Cookieverses, and Alternate Grandmas have a resemblance to Grandmapocalypse Grandmas. Clearly someone is importing their cookies and grannies from other players.
Additionally, historians deem cookies produced from time machines to be "unfit for human consumption". These cookies are explicitly being taken from the past, so they might very well be the cookies you were making at the very start, long before you ever started bending natural law to your whims.
"The Digital", "Rewriting history" and "It does matter!", the achievements earned for having 200 cursors, 100 time machines and 100 antimatter condensers respectively. Because of how buildings become more expensive as you buy them, it takes trillions upon trillions of cookies to complete a single one of these achievements, let alone all of them. By extension, there's "Centennial", awarded for having 100 of each building. Version 1.0375 similarly adds "Ruler of the Ancients," which requires 200 grandmas (made slightly easier due to their quadratic growth in production at the high end).
"Let's Never Bake Again" has become this through subtle mechanics changes and Rank Inflation, for those who haven't had this achievement grandfathered in. In versions prior to 1.038, LNBA required a player to bake 1,000,000,000 cookies per second; in versions without Heavenly Chips and Antimatter Condensers, this took a fair while, but was manageable thanks to the Frenzy and Elder Frenzy buffs that multiply one's CPS by 7x and 666x, respectively. As of 1.038, however, LNBA requires the player bake 1,000,000,000,000 cookies per second without Frenzy buffs. This is difficult to impossible without roughly 4,000 Heavenly Chips due to Adam Smith really hating your guts. "How?" has received a similar spike in difficulty, from requiring 10 trillion to 10 quadrillion cookies baked all time, but as getting 100 Antimatter Condensers already requires triple this increased amount, it is still fairly easy to obtain.
Sextillion Fingers is That One Upgrade. The upgrade itself costs a mere 500 trillion cookies, but unlocking it requires you to buy 240 cursors which, because Adam Smith Hates Your Guts, will run you just barely under 37 quadrillion cookiesnote 36,938,706,618,205,200 cookies, to be exact. This is roughly the number of cookies needed to get all other unlocks and every single achievement, including the obnoxious ones listed above, so Sextillion Fingers is like 200% Completion. Worst of all, it's not even remotely worth the effort—it'll only increase your cookie rate by a few percent, at best.
Grease's Cups is even worse. You need at least 2000 Heavenly Chips to unlock it, which requires you to forfeit over 2 quintillionnote 2,001,000,000,000,000,000 cookies by resetting. note Slightly less difficult than it sounds, as having HC's from previous resets will dramatically increase cookie production; a 100 AC endgame with no HC's will generate around 70 billion CpS, but with 1500 HC's, that number increases to around 400 billion CpS. Further, the 2 quintillion need not be earned in the same playthrough, as the count is cumulative across soft resets.
The last Spooky Cookie is another one, due to the mechanics for unlocking them. Whenever you burst a Wrinkler, you have a 5% chance of unlocking a Spooky Cookie. However, the game then randomly chooses a specific Spooky Cookie to unlock, and if it's one that you already have, you get nothing. This means that if you already have six Spooky Cookies, you have a one in seven chance on top of the one in twenty chance when you burst a Wrinkler.
Want the maximum achievements for Grandmas and Cursors, and all of their upgrades? Gotta say tough; this will run you 4,613,212,048,816,833 cookies. Without heavenly chips, good luck.
The Speed Baking achievements, which require you to get to 1 million cookies in a very short period of time. Speed Baking III in particular requires 1 million cookies in less than fifteen minutes, and its reliance on certain types of Golden Cookies makes it a Luck-Based Mission. Oh, and you can't use Heavenly Chips.
Bicentennial, as of 1.041, has become the new achievement beyond all others. To get it, you need to have built 200 of every building in the same playthrough. Combined with the price of the new Prism building (which thankfully eventually upgrades to give ten times the production of the Antimatter Condenser... but costs 12.5 times the starting cost of the Condensers), and you're literally looking at spending hundreds of sextillions (over 10^23, in scientific notation) to get everything (the 200th Prism alone costs over 56 sextillion).
Briefly, version 1.0418 presented a host of achievements for baking at least X number of cookies solely through Y building, almost all of which qualified - the numbers were not only extremely high, but they didn't factor Heavenly Chip production at all. Moreover, resetting the game also reset progress towards these achievements. Best case scenario, they would take a couple of weeks, while the worst - Magnum Opus for earning 10 quadrillion (10^16) cookies solely through Alchemy Labs - was estimated to take more than 3 months of continuous play solely optimized for earning it. Relaxed as of 1.0452, when multipliers did affect how quickly those were earned while also including upgrades to each building type as well as a new kitten upgrade to radically boost cookie production.
Version 1.0418 still has quite the legacy for headaches. One, the price of prisms went up 50%, meaning that Bicentennial now costs north of 600 sextillion cookies to earn, to the chagrin and despair of those aiming for it. But even that pales compared to Dr. T, requiring the purchase of 400 cursors. Going for this requires spending over 100 septillion (10^26) cookies just on cursors. Even with upgrades across the board that could over quadruple production once unlocked, the sheer number required is staggering.
As of the Easter update there is now an achievement for resetting with one octillion cookies baked, outstripping even Dr. T by a factor of about 10. Worse yet, there's even one for resetting with one nonillion, but that's a shadow achievement.
Another update, another addition to qualify. The eggs in the Easter update are similar to the Halloween cookies both in how they spawn (a chance whenever you click a Golden Cookie, a Wrath Cookie, or pop a wrinkler), but also in that it will randomly choose one from the list of eggs to unlock - if you already have it, you don't get an unlock this time. And while you only had seven Halloween cookies to worry about, there are twenty eggs to get at Easter, eight of which are rare and thus have a lower spawn rate to begin with.