I think sometimes they are. I think its the World that teaches us that mistakes always haunt you forever and ever and there's never any "un-doing" of what's been done.
Personal example: I committed a crime against a family member. I should have been either convicted and given some sort of punishment--as a first time offender for a "High Misdemeanor," it probably would have been a lengthy probation period-or I should have been sent to a state mental hospital for several years. Neither happened. I was allowed to go to a Christian Rehab in another state for 1 year, do some additional counseling and volunteer work, and get the charges dropped and the arrest record expunged. I get along with my family better now than I did before. People in my small town, who never really liked me to begin with, now leave me alone.
I didn't "undo" the mistake--that's been done--but God allowed me to escape the usual consequences. Christian Rehab (it was more of a work camp, but whatever...) certainly wasn't fun, but it was helpful. I suffered for my mistake, but in a limited fashion.
Similarly, God sometimes reverses the ill effects of sin for others, too. At my Christian rehab, there was a great counselor who had been addicted to cocaine (mainly used the IV route) for 30+ years. At one point, he was diagnosed with Hep C. A couple years later, his liver was in good condition and there wasn't a sign of Hep C. I've read about homosexual and drug-abusing Christians who tested positive for HIV and for whom death seemed imminent. One man I read about (don't have a link, sorry) has been HIV+ for over 15 years and has never needed the antiretroviral medication. Did God reverse ALL of his mistakes? No. But did God make him suffer ALL the consequences? No. This man gave up his promiscuity and homosexuality for Christ, and God saw fit to, if not heal his HIV, then at least stabilize it for a long time, so AIDS isn't a major concern for him.
Another personal example: I used to pop pills like crazy. Then I quit. Before I was a Christian, I had to go to a mental hospital b/c I had a psychotic break. This particular mental hospital does brain scans, since they're connected to a medical school (and also because I'd been ht on the head during a botched mugging the night before I was put in the hospital). My brain is apparently **fried**. It was so bad that when I woke up from sedation (they had to sedate me b/c I was being violent), a nurse said to me "I've seen your brain scan. I bet you have a 75 IQ. You're supposed to be a vegetable." My psychiatrist said that I "seemed to have some sort of 'transcendent intelligence'."
To be honest, I wasn't very smart at that point. I was pretty burned out. But after Christian Rehab and some mental health services, my mental functioning got better. My last counselor told me--without me asking, mind you--that I was "definitely above average" and that I should do something "mentally stimulating." Did God let me fry my brain w/o any consequences? No, definitely not. I went through years of being stupid. People would talk about how stupid I'd made myself (small town, everybody knows everybody's business). I was soundly rejected by my peers and unable to do much of anything. But in the end, I was restored to health and although I'll never get those years back, I have been healed and--just as important--I've learned my lesson: drugs are bad, mkay.
...No. They are heal-able, they are changeable, they are learn-able...but most of all, they are forgivable. 
we can be thankful for this because with out it, we have no hope for the "mistakes" we have made, we are making, and will make.
