What to do in dealing with scientific mistakes in an article?
In this kind of scenario, write a letter to the editor. To quote:


  In academic publishing, letters to the editor of an academic journal are usually open postpublication reviews of a paper, often critical of some aspect of the original paper. The authors of the original paper sometimes respond to these with a letter of their own. Controversial papers in mainstream journals often attract numerous letters to the editor. Good citation indexing services list the original papers together with all replies. Depending on the length of the letter and the journal's style, other types of headings may be used, such as peer commentary. There are some variations on this practice. Some journals request open commentaries as a matter of course, which are published together with the original paper, and any authors' reply, in a process called open peer commentary. The introduction of the "epub ahead of print" practice in many journals now allows unsolicited letters to the editor (and authors' reply) to appear in the same print issue of the journal, as long as they are sent in the interval between the electronic publication of the original paper and its appearance in print.


Example of such a letter, and the authors' response to it (and other letters criticizing their original results).