Mistake #1: Going too Big
Mistake #2: Planting in the wrong spot
Mistake #3: Overwatering or Underwatering
Mistake #4: Cross Pollination
Mistake #5: Planting too late
Mistake #6: Not weeding
Mistake #7: Planting too close together
Mistake #8: Planting too deep or too shallow
Mistake #9: No drainage
Mistake #10: Not keeping track of what you planted
Don't let this list intimidate you, if you are a new gardener, just use some common sense and ask friends or family who garden, other then the mistakes mentioned there really is no right or wrong way to garden, you can ask 10 gardeners how to grow a tomato and I guarentee you will get 10 totally different answers, and they will likely all work. Happy gardening!