Player's stupidity? Does it matter?

While I love stories of parties getting wiped out for stupid mistakes (even when I am part of it), if think the following is the problem:

How do you define what is stupid behavior?

Most of the examples I remember about players being punished for stupid behavior are from the 90ies when realism was en vogue with many gamers and most of them (at least in my circles) did not really reflect much about genre conventions.

So when you approached a situation the way a character in a book or movie would do, you often had a 50% chance that the GM would suddenly realize how ridiculous over the top action movie manoeuvres really are and punish you for it. 50% of the time you would be able to pull it off, because the GM was in the proper mood from a coffein high or a Monty Python joke.

I have to admit there was another factor that made me see players' stupidity as pretty relative and depending on the GM's mood:

In one of my groups back then the situation was made even more transparent, because we were a bunch of 16 years old hanging out with some 18 years old. Our young GM would think that most of the older guys' ideas were teh awesom!!1 even if they were as stupid as the other players' ideas.
This made it pretty obvious to me that judging stupidity is highly subjective.

Not to talk about the situation that with some GMs your well laid out plan fails most of the time, because it would destroy the pace of the adventure to have the big success so early on, while your shitty back up plan probably will work, because the session has to end soon.