Quote Originally Posted by Evil DM Mark3 View Post
It tends to upset the player who took time and effort to craft careful strategies or delicate plans to find they don't work because the DM has decided to change things, particularly if they weren't told about the change beforehand.
A good DM will tell their players about any special changes to the setting before the game starts. If I've removed elves from my setting, I better tell you before you decide to roll up that elf PC.

At the same time, a good player shouldn't assume the DM won't make changes and probably shouldn't base their plans on things without confirming with the DM that they'll be valid in the upcoming adventure. If you plan on making a stealth-based PC in my game, make sure you ask about and understand any modifications to the stealth rules I may have made.