RPGMemes

from my experience i'd say players expect that certain monsters behave a certain way,

e.g. trolls regenerate, displacer beasts teleport around and a vampire can suck your blood, rather than a bear has 16 strength 60hp and so on and players value consistency, so if a kind of monster had no fire resistance the first time they've encountered it they should not have it next time only because the wizzard destroyed 5 of them with a fireball.

some players read published adventures though and get irritated if a game master changes stuff, but those guys can run their own game where everything is straight from the book rather than adapted to the group.