A popular (and relatively new from my old-timer point of view) GM meme goes something “Always say ‘yes’ or ‘yes but'” with a strongly implied never say no. Personally, I think this is some of the worst GMing advice I’ve ever heard as there are times when saying anything but “no” is going to seriously hurt a game session or even a campaign.
For example, players who want to play characters who do not fit the campaign premise or the campaign’s setting need to be firmly told “no” as allowing characters who do not fit the campaign is going to make the campaign less fun for the GM and probably less fun for at least some of the other players. As a GM you should not feel you are somehow obligated to accept every character concept a player comes up with or to change the campaign setting or premise to compromise with a player. If a character does not fit the campaign premise, the setting, the play style of the rest of the group, or uses rules that you as GM do not want to use, don’t feel bad about rejecting the character outright. Do not feel an obligation to compromise with the player over a poorly fitting character concept unless such a compromise does not interfere with the campaign.
Another example, some things are simply impossible to do as they violate the “physics” of the campaign world. While a character running fast enough that time flows backward might be acceptable in a high-powered comic book superhero game, it’s going to be simply not possible in the average fantasy or spy campaign. While this is an extreme example, I found over the years that players often come up with ideas that are just impossible — and they need to be firmly told “no, you can’t do that” when they do. Saying “yes” or even “yes, but” when something is actually impossible leads to all sorts of problems. A more reasonable example would be a character with a lot of skill in diplomacy trying to use a skill roll to talk an NPC enemy ruler into doing something that no one in their right mind would agree to (like turn control of the country’s army over to the enemy PC). It doesn’t matter how skilled one is in diplomacy, no sane person is going to agree to something like that unless he is under the influence of drugs, magic, blackmail, or the like so the character is effectively trying to do something that is simply not possible: no matter what they roll, the attempt should fail. Some cases where people see “broken rules” are really fine as the rule in question is only broken if the GM never says “no, that’s impossible.”
Saying “yes” or “yes but” when you can is a good idea. However, when “when you can” becomes “always”, I’ve found it to be a very bad idea. Think before you say “yes” (or before you say “no” for that matter) — don’t let some “rule of good GMing” (that someone made up) force you to say “yes” when saying “yes” is actually a bad idea. This is even more true in old school games.