While I have never had a bad trip, I tend to argue that a bad trip is more likely to occur if you are in a unstable inner state and or your projection of the drug or the possible experience of using the drug is unstable.

Then again, I think your question comes down to semantics. I have heard friends say a bad trip is their best trip. But anyways and above all, I want to say, don't use labels of good, bad, etc, and just experience the experience.