Humanity, as a whole, has a tendency of getting excited. When we get excited, we lose our rationality and just tend to act. This is shown to us constantly in works of fiction with things such as depression and rage. The hero, generally having just suffered a great lost, goes out after the big bad in a fit of rage and passion. Next thing you know, things are worse than ever because of the mistakes that were made while impassioned. However, it isn't just negative emotions that you need to watch out for.
Excitement Can Be Positively and Negatively Fed
First off, you need to be aware that passion can come from good and bad emotions. Whether it is love, depression, happiness, or rage, once we get gripped by emotion we tend to lose sight of logic and rationality. Sure, the symptoms and the way it happens might be different, but the end result is the same. This means you have to be careful as a GM and as a player. Why? because often times those ideas that we love and just can't wait to do, have consequences - obvious consequences - that we just don't see. In short, what I'm trying to say, is being excited makes you dumb.
Thinking Things Through
The ability to stop and think things through is an invaluable skill in every walk of life, and unsurprisingly it can pay dividends whether you're GMing, playing, or designing an event or game. Unfortunately, it is something that we all forget to do from time to time, no matter how many times we tell ourselves to do it. It's just part of being human, but you get excited about an idea, you can't wait to show it off and get people involved in something...but you don't think it through. You don't look at all of the effects what you're doing will bring, the alienation it might cause, or any of another dozen effects that it could have. Then, you get blind sided by those very effects that you didn't think would even be a problem.
Unfortunately, there isn't really a defense against this once it happens. The best thing you can do is admit you missed stuff, take a minute to figure things out, and go from there. Depending on the effects, it might be worth running with them, but if it is hurting your game, you may need to apologize and wind it back. The defense that you can do, is just to be vigilant for this. It is one of the traits that separates the good GMs from the truly great ones.
Now, sometimes you'll mess up on this, and that is fine, but here are some ways to help mitigate those from happening.
1) Don't plan game when you're angry or depressed. Planning out fights when you're in a destructive mood can quickly turn into you wanting to destroy your players with a fight, even if non-intentionally. The next thing you know, you've gone a bit further beyond what your group can handle, and when you execute it in the game...well, you also execute the group. It might even be worse if you never even notice this happening. I mean, it looks like it should be a fair fight, you crafted the encounter carefully before the game, but in actuality it is skewed to one side just enough that if the PCs aren't rolling ridiculously well, they're going to get butchered.
2) Don't expect good rolls to save the PCs. This is an extension of the one before, but sometimes it is possible to get drunk off your PCs success, and then skew things too much against them. The end result? Generally a TPK the second the PCs' dice don't hold up for them.
3) If you love an idea for the idea's sake, then run it by someone else. The internet is full of boards for lots of games where you can talk with other GMs, bounce the idea off of them. This happened with me recently. I was all excited for a big battle in my Deathwatch game, where the Dark Angels would attack a Watch Fortress. I ran it by the Deathwatch forums, and immediately people pointed out how all encompassing the consequences for that would be. The end result, was I modified the encounter and came up with something better, and without game breaking consequences.
4) Take a look at it from a player's perspective. This can be a hard skill to learn, but just picture the event, don't think of all the details you also know, and try to picture how you would react as a player to it. If it seems unfair, then you may want to fix it so it seems more fair.
Just keep those things in mind, remember to think things through, and hopefully you'll be able to avoid causing more damage to your game then you wanted to. Just, if you do mess up, don't beat yourself up too much about it. Learn and move on.