Mike Warnke once said that the gruesomer your testimony, the more people want to hear it.
He does have a point. When you tell people that you were born into a Christian family, went to church your whole life, and got saved at an early age-- nobody wants to hear that. That's boring. That's not a testimony at all. They want to hear about the evil, depraved life you led and what a radical change Jesus made in you. Now, that's a testimony.
This leads to a tremendous temptation to spice things up a bit. How easily then do the elderly aunts who raised you after your parents died become a series of foster homes. Then that series of foster homes becomes a series of abusive foster homes. Then those abusive foster homes become crack house foster homes, and so on..
The occasional childhood spanking becomes a childhood filled with abuse. The decision to move out becomes getting kicked out onto the street. A bachelors degree turns in to a masters, then a doctorate, then two doctorates. One war wound becomes two, then five. The purple heart becomes a Silver Star, or a Congressional Medal of Honor. The typical teenage rebellion becomes a life filled with drugs, pornography, Satanic cults--complete with bloodletting and human sacrifice.
Oh, these people usually are well meaning--the Jim Bakkers of the world notwithstanding. They just want to make an impact. They want to lead people to Jesus. The problem is, the truth will always come out. And when it does, even the slightest embellishment to your testimony will completely destroy your credibility as a Christian witness. Even worse, people tend to judge Jesus by what they see in us--in you and me. They don't always get that the failure of the Christian does not equal the failure of the Christ.
So, if you truly want to make an impact, it's best just to be honest all around. If you don't, you may end up doing more harm than good.