is not wanting to know a twist/ending to a work of fiction stupid?
No. However, I think the original intent of the thread was to explore if there are exceptions to this. Particularly in works based on historical events, most of which are common knowledge. Knowing that the british lose in a movie about the american revolution should not be considered a spoiler.
The other addendum expressed is if a movie or work is old enough and referenced enough to be common knowledge. Though, this one, I will admit, can be hard to define. Nearly everyone knows the who Luke's dad is, or the state of Bruce Willis's liveliness, but if I knew someone who honestly did not, I would not want to spoil such thing for them.