Too Many Requirements, Not Enough Resources.

History has proven that the people who think they're invincible are the ones who usually get clobbered. Game development is no different, so it's important to make sure that you keep your head out of the clouds. Think long and hard about what it will take to create each aspect of your game. Don't ask yourself, "Can I do this?" Instead, ask: "Do I absolutely, positively, without a doubt, know that this is within my grasp?"

I'm not saying you should be forever content with making 16 color Space Invader clones. It's perfectly acceptable for your team to have a couple of "challenging areas" in your designs. But don't overdo it – make sure that by the end of your design phase, your team still considers your project "easy."

Also, just because something isn't technically hard doesn't mean that it's not a lot of work, that is, just because it's easy to make one level doesn't mean it will be easy to make ten levels. Numbers have a way of tricking developers into thinking something's easier than it is. Saying "this game will have 5 levels, with 5 different enemies on each level" makes it sound easier to do than saying "this game will have 25 different enemies." Be careful about such things, and make sure you know for sure what you're dealing with.

"I Can Do Anything In a Week"