If thinning is needed, use the thinner recommended on the label. If strong solvent is added, uneven gloss, wrinkling, and dissolving of previous coatings are only a few of many problems that can be caused. All thinner must evaporate from a coating before a film can begin to cure. Solvent is normally added to change consistency for different application methods. Oil base products can have the dry time slowed down dramatically.
Latex paint can have film formation interrupted before it has totally formed.
Many different things can happen. Most of the time there may be no visible sign of a problem, but in almost all cases there will be some kind of long term affect, reducing the life of the paint job. The more gloss a finish has, the less dirt will adhere to it. The more gloss a finish has, it will most often have a harder and more washable finish, and dirt will wash off easier. Lower quality coatings do not wash as well.