I found a most interesting article on this subject today. In my opinion it is a Must-Read for every amateur gardener, especially those who grow beans, tomatoes and dahlias.
Here is a link to it: Detecting persistent herbicides in compost
Unless you are very scientifically-minded, you will probably want to read only the Executive Summary at the beginning and the Conclusions at the end!
The general gist of the article is that the study found that a very high proportion of commercial composts are contaminated with residues from weedkillers, and these residues have a very adverse effect on vegetables and flowers.
Please do whatever you can to raise the profile of this issue, and lobby for the use of clopyralid (and aminopyralid) weedkillers to be discontinued.