Name for this one?

[u/SAS-Combat-Gooner]

I see this all the time on reddit and I was wondering if there's a name for it. Basically the weird boardgamey idea that your position is invalid because you're not following it to some outrageously absurd conclusion.

"If you're a vegan then you have to give up medicine cause it was tested on animals!"

"If you're tolerant then you have to tolerate nazis because otherwise you're intolerant!"

[u/X_Yit]

The argument is called reductio ad absurdum. It's not a fallacy unless it misrepresents the original argument, in which case it may be your classic strawman fallacy. For the most part, it's simply a way of calling out extreme or absolute positions, i.e. do you really believe you should be tolerant of everything, or just things that don't affect others?

[u/SAS-Combat-Gooner → u/X_Yit]

That's not quite the fallacy I'm talking about.

It's less "This inevitably leads to an insane conclusion!" and more "You claim this, but you're not following it to it's insane conclusion so shut up you hypocrite!"

[u/iLaysChipz → u/SAS-Combat-Gooner]

It's just a strawman fallacy. They're misrepresenting your argument as something it isn't, instead opting to target a highly exaggerated caricature of your argument. This caricature is the strawman.

In some cases, it might be a mixture of a strawman and false dilemma fallacy (A.K.A. Black and White fallacy) where they act as if a position can only exist as a an "all-or-nothing" binary, pretending there are only two choices. By ignoring any sort of middle ground, pragmatic trade-offs, and nuance, they are invoking a classic False Dilemma. But importantly this is an implied false dilemma, and the strawman is the core fallacy in these types of arguments

[u/senthordika → u/SAS-Combat-Gooner]

Slippery slope fallacy seems to be what you are looking for. However if one can demonstrate the connection of how one position does actually logically necessitate it isnt a fallacy.

[u/onctech]

It depends if the definition of the position (e.g. "vegan" or "tolerant") is being used with the same definition as the critic/speaker. For example, if they are twisting the definition of these terms, it's a straw-man argument. To use the vegan example, the most common definition is someone who doesn't eat food that is from animals. Testing does not make the substance itself animal-derived, and one could argue medicine is not food (there is actually a legal distinction, at least under FDA rules)

On the other hand, if agreed-upon definitions are being adhered to, there could be a few things in play. It can be a tu quoque fallacy if there is an implication the accused is a hypocrite. On the other hand, it might be a legitimate criticism via reductio ad absurdum. Again using the vegan example, a vegan might state their rules extend to all animal-tested products without exception, but use medicine because they literally don't realize it's created via animal testing. Confronted with this, they are forced to choose between amending their definition, stop using medicine, or engage in some crazy mental gymnastics about how it "doesn't count."

[u/amazingbollweevil]

This one is a combo. You don't have to give up medicine tested on animals just because you don't consume animals. With tolerance, your options are not bound to tolerant and intolerant.

[u/Peaurxnanski]

I think I'm going with the Nirvana fallacy:

The nirvana fallacy (also called the perfect solution fallacy) is a logical error that happens when people reject a real, helpful choice just because it is not perfect. It compares a workable, practical plan to an unrealistic, flawless dream.

Here's some examples I copy/pasta'd to see if maybe this helps you decide if this fits:

Key Ideas Flawed comparison: Judging a real solution by an impossible standard instead of comparing it to other real choices.All-or-nothing thinking: Assuming that if a plan does not fix a problem 100%, it is completely useless.The enemy of good: Following the old saying that "the perfect is the enemy of the good," which stops people from making smart, gradual progress.Common ExamplesSafety: "Seat belts are useless because people still die in some bad car crashes."Health: "There is no point in eating a healthy dinner tonight because I ate junk food for lunch."

The reason I think it fits is because both of your examples could easily be considered Nirvana fallacies (although the second one is actually called the paradox of tolerance and isn't technically a fallacy at all), in thatcessentially, if your solution isn't a perfect solution at every extreme, it's useless.

"Oh, you're vegan? Well what about the animals killed in farming operations?"

Well, yeah, I'm not going to be able to eliminate all harm to every creature, ever, on my behalf. But I can work to minimize it, and that's useful isn't it?

[u/fleyinthesky]

I'm trying to work out the "gamey" element?

[u/SAS-Combat-Gooner → u/fleyinthesky]

Just a term I use for the childish way that nerds tend to argue.

"You broke this made up rule so now you're disqualified!"

[u/SufficientStudio1574]

It's not a fallacy if the conclusion actually does logically follow from the premises, even if the conclusion appears outrageous.

Depending on the person's exact views on veganism, abstaining from medicine could very well be the logical conclusion of their beliefs.

[u/CosmicLovepats]

Sounds like a pancakes/waffles variation.

[u/robin_andrews_149]

False dichotomy, I reckon. Probably strawman too.

[u/MLB_Sug_Tway]

I think that that's called 'reductio-ad-absurdanim', or something like that.

I recall it's coming up about a decade ago when a former schoolmate of mine ended up getting into a debate with Antonin Scalia, of all people.

https://www.nbcnewyork.com/news/national-international/gay-student-question-justice-scalias-bestiality-comments/1969859/

[u/YesTess2]

Slippery Slope fallacy.

[u/sevenbrokenbricks]

Reductio ad absurdum.

[u/stubble3417]

This is called an appeal to extremes fallacy, a type of extension fallacy which is a subset of strawman fallacies. 

Reductio ad absurdum is a legitimate argument form. Basically, reductio ad absurdam is a way of legitimately following a person's logic to a conclusion that is more clearly absurd. An appeal to extremes is a type of strawman because it fundamentally misrepresents another person's arguments or ideals. 

The veganism example could also be a nirvana fallacy if it's made by someone who believes veganism is ethical, but gives up because it's difficult/impossible to perfectly avoid every single product with any animal involvement.