I have encountered this reasoning quite frequently:
Somebody posits the hypothesis that an event X can happpen. A recent example I encountered was "vinegar and salt in the boiling water make eggs easier to peel afterwards", but I have seen many more. Another person imagines a way in which it could happen, judges that way to be impossible, and claims that X can never happen. In that example, the reply could be "salt and vinegar cannot cross the shell, so they will not make the egg easier to peel".
I have seen two ways in which this argument can be false. First, the replying person might just have wrong knowledge (maybe vinegar or salt can pass the shell?) - but I don't think this is a problem of the reasoning process, it is just correct reasoning based on a false assumption. What I find more interesting is that frequently, there can be other mechanisms for X to happen - maybe vinegar doesn't have to pass the shell to make the egg peelable? - but the person making the argument overlooks that possibility and goes from "the mechanism for X that I have in mind doesn't work" to the conclusion "X is completely impossible".
Another example would be people who insist that placebo does not help against pain, because per definition, it contains no chemically active substance that acts on pain receptors.
Has this flaw in reasoning received a recognizable name? I am asking specifically about fallacies as they apply to a formal reasoning process, not a general reason why arguing against vinegar in egg water would be a wrong position to take. (In fact, I don't even know if vinegar in egg water has any effect).