Update becaue I don't want to get into a semantics fight: No system is robust to handle ALL situations. No GM can think of all player interactions ahead of time. Sometimes you have to wing it, but do so in the spirit of the rules. It is not cheating the players in the classic sense, it's keeping the game going by understanding the mechanics behind the rest of the system. It's common. I don't understand people who say they don't do this, but then talk for hours about house rules and home brew. I'm just talking about doing it faster and on the fly.
Been a GM for a while? Let me ask you a question, do you cheat? Do you cheat a LOT to make the game flow nicer when the system get in the way? Do you learn a rule framework well enough that you can make instant judgement calls and situational rules breaks just so it's more fun for the group? Have you figured out the right balance of just making stuff up but giving the players a sense of impact in a fictional world?
No? Then you must be new, or a stickler to the By the Book ideology.
For the rest of us, it's all about system/setting emulation and the fine art of "close enough". Yep, you are literally making stuff up that doesn't exist on the fly just for the sake of the game.
What am I talking about?
GM's who spend a LOT of time reading, playing a game system being to internalize the logic of the game. We see the overall framework the game is hung form that the developers used when creating it. If need by once that level of familiarity is hit we can make up foes, attacks, effects on the fly that are "close enough" to the game that players don't notice or care that it's on the fly nonsense and they enjoy that session's twists and changes. What you make isn't rules legal, but fakes being so well enough the players don't notice...much.
Some games have their framework naked for players. GURPS and HERO are two prime examples. Once you learn the core system you learn the framework and it's easy to bs a foe or event. Other games are more obscure.
D&D 3.X/Pathfinder is a classic example. Problem events like Save or Suck/Die spells, CoDzillas, and such are because there are optimal choices buried in the fluff and charop (character optimization) players and GM find them and can abuse or use them to their liking. This is why people talk about the "sweet spot" of 3.X play being around the 5-10 range level wise. it's in that cozy middle ground where the player options and GM options are mostly balanced and you haven't hit the rocket tag, instant death situations.
When playing ANY game system a GM has to dedicated a lot of time learning the ins and outs of how system interacts with players. GM's develop signature system and mechanical sub-systems they are cozy with.
Like in my case I grew up deep in the world of GURPS 3rd Edition. I 'grok' generic or generic enough system. This translates into me deconstructing games like D&D, L5R, and such mechanically so I can have a feel for the break points in the logic. Games with more crunch like Exalted, Star Wars EoTE, Shadowrun etc are more my cup of tea. I have serious issues with more narrative games like FATE, Cortex+, and the like because I don't see where the mechanics plug into the characters. I loose my world emulation feel and it's VERY HARD for me to wing it in those games. Bugs me to no end. Dungeon World is a cozier middle ground because of the naked mechanical subsystem at work behind it.
Other GM's have their own tastes and systems of choice. If you find a GM who's run for a long time and they have the right mix of tight rules calls and decent bs hiding going on, they you should be in for a great time.
I'm cultivating that skill set with Pathfinder and OSR games.
I have it DOWN with games like GURPS/HERO.
I'm loosing it with games like Exalted and NWOD from lack of use.
Good GM bsing on stats is a skill you use or loose. So my GURPS is rusty but my HERO is more solid. Given time I'll have Pathfinder mastered but will need refreshers in other system.
I'm going to keep learning mechanics and pulling stuff out of ether for my players because it's fun.
learn the mechanics and the system logic so you are better at making stuff up on the fly. Players will thank you for it if you can do it like any good stage magician.
Smoke, Dice, and Mirrors.