Well I'll tell you, I have heard the story all too often. Not only does the use of these machines turn out terribly quite often but they leave soapy residue in your carpet. The vacuum on most home machines is just not powerful enough to pick up all of the cleaning solution on the floor and leaves enough water behind to cause serious damage. That causes a mildew smell and discoloration. Well I'm sorry to tell you but even a professional can't remove the damage that has been caused.
Once the carpet and, very likely, the pad become saturated and sit overnight that musky mold smell starts forming. The smell may go away from the air but it will always be in your floor's padding. It may be cleaned very well but the smell will remain in the backing of the carpet at a minimum. Doing your own carpet cleaning is not usually a good idea because of these results.
The professional floor care providers of Webster know about certain rules that come with carpet cleaning. I will share a few with you. One thing, you never over wet the area, two, rinse all cleaning solutions out thoroughly, and three, get it dry as fast as possible.
I have explained above how over wetting carpet can have negative results. If a cleaning solution is applied to heavily it can cause the pad to get wet and result in musky smells. Rinsing out all of the cleaning solutions while steaming with a truck mounted unit is important for our health. If allowed to contact skin the chemicals can be very irritating given time, especially with children. Also soap residue is sticky and will cause a high resoiling rate. Your cleaned areas will get dirty faster than you would believe. It is very important to get the carpet dry as fast as possible because doing so prevents the growth of mildew, mold, or bacteria.
This is solid knowledge and advice from an experienced floor cleaner. I have seen it with my own eyes. I have talked to many moms who all had the same story about their carpet cleaning machine and how spot after spot they got overwhelmed.