Supplements can and have been contaminated by all manner of things. Heavy metals, pesticides and other pollutants are the most common. But let's be clear, say what you want, but supplement companies don't usually "put" contaminants into sups. There is a complex production cycle for the ingredients in sups and they can be contaminated somewhere in that process. So it would be more fair to say that some companies don't always verify the purity of the ingredients they use.
Plant ingredients in sups can be contaminated with pesticides sprayed on the plants or by heavy metals found in the soil. Minerals can be contaminated by heavy metals just coming out of ground they are extracted from. Some supplements have even been found to contain dangerous prescription medicines. Creatine has been a big problem in the past with the worst (to the best of my knowledge) being the stuff coming out of China.
And yes some supplements don't meet their label claims. But if they claim an ingredient of a certain amount on the label they are required to meet those claims and there are many companies with actions being taken against them. There are also many companies making fraudulent claims (especially diet sups) that also have actions.
But yeah it's buyer beware. You have to do your best to investigate the sups you use and buy from companies you trust. The good news is there are plenty of good companies out there that are trustworthy. You can always try to obtain an independant essay from a sup company on any of their sups you buy. If they won't provide one that may be a clue.
There is also consumerlab.com which conducts independant tests on label claims and contaminants. Some results are viewable for free but for the good stuff you have to pay a membership fee. May be worth it for someone who uses a lot of sups.
There are other ways to get a pretty good lowdown on certain things. Maybe not always definitive but certainly better than no info at all. You just have to be diligent.
BTW, a lot of people have helped you with these sup questions. Sometimes a simple thank you can be nice before you move on to the next question. Call it character building
But at least don't continually start threads and then abandon them. I for one would at least like to no that my response was even noticed.