Are there any nouns that describe an idea which on its very surface seems plausible but with some (usually minor) investigations can be deemed false or much less plausible?
If I were an Italian living in the US, and an Italian family moved into the town I lived, a friend might ask me "oh, you're Italian, do you know them?", but they may think right after saying it that there are millions of Italian people therefore although after a short amount of thought they considered their idea plausible, with a tiny bit more thought, the idea became less plausible (although still theoretically possible).
Chess gives another good example. Suppose you could capture your opponent's queen (their most valuable piece), but after a couple of seconds analysing, you discover that your opponent can check mate you (win the game) in 1 move if you are distracted by the capture of their queen. This move that captures the queen looks brilliant but is revealed to be extremely bad after just a short amount of thinking.
What I've thought of
seeming or said to be true or real but very possibly not true or real. E.g. The ostensible reason for his visit was to see an old friend.
But this is an adjective, rather than a noun.
also, disapproving : reacting in a readily predictable way
This is getting closer, since at least 'knee-jerk response' is a noun. The problem with knee-jerk response is that it implies that the idea was jumped to quickly, whereas sometimes plausible but incorrect ideas are a product purely due to lack of thought, rather than a lack of time to think.
A lack of thought could be because someone spoke too soon, but it could also be because they simply never thought more deeply (i.e. even though they had time), which makes it less of a knee-jerk response and simply an incorrect one.