On the spot fallacy
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Logic and rhetoric
An on the spot fallacy (OTS) is a logical fallacy that occurs when a debater is considered wrong (or even incapable of having an opinion) if they cannot recite specific data or technical minutiae on some topic. The fallacy asserts that one must be an expert on a topic in order to discuss anything related to it (and, at that, an expert with flawless memory).
The fallacy often occurs during a Gish gallop -- because people are unable to respond to so many arguments at once.
The fallacy is a conditional fallacy, because people are expected to provide evidence for their position; this fallacy occurs when people ask for evidence that's unreasonably in-depth.
- drag-you-into-the-weeds fallacy (alternately "drag-you-into-the-weeds-and-strangle-you" fallacy)
- minutiae trap
OTS is a mixture of credentialism and appeal to authority (you're wrong because you don't know absolutely everything about an issue), shifting the burden of proof (you have to prove that something is absolutely true, while the opposite side has no burden to disprove what evidence you present) and moving the goalposts (by demanding increasingly overspecific replies to a question that's already been answered, until the replier fails to be more specific, at which point the original question is considered unanswered or incorrectly answered).
The issue isn't just that it's practically (though perhaps ultimately not technically) impossible to meet the excruciatingly high burden of proof they request, but rather that it's unreasonable for you to have everything available immediately. Even experts don't carry around stacks of all the collected proof for their position.
In retort, users of OTS almost always have some factoid (often a one single proof-style argument) or discredited research at the ready. It's also used as a springboard into several other fallacies or fallacious arguments, because once you've fallen for the trap, they can throw their fallacious spells at each thing you cite.
Some people (especially creationists) abuse the fallacious-ness of OTS to say that, just because I don't have an explanation for why I believe what I believe now, this doesn't mean that no such reason exists, and thus there's no reason for me to reconsider my beliefs. Taken to an extreme, this argues that, because it's possible that a good reason for a belief exists, that belief is justified. This, too, is fallacious. There is a difference between being unprepared to defend a position (which you have good reasons for holding, though just not on hand at the moment) and holding a belief for literally no (or indeed some rather poor) reason.