The longer the chocolate is kept at the working temperature, the more form 5 cocoa butter crystals are formed. And the more the chocolate is stirred, the more form 5 cocoa butter crystals are formed. Therefore, you may experience that the chocolate gets very thick over time, just as the chocolate can also become thick if you stir it a lot. One then speaks of the chocolate being over tempered or over crystallised.
Both time and agitation are catalysts for the formation of form 5 cocoa butter crystals in the chocolate. If the tempered chocolate has been kept at working temperature for a long time, or if it has been stirred too much, it can lead to formation of so many form 5 cocoa butter crystals that the chocolate becomes over crystallised/over tempered.
If the chocolate becomes over crystallised, the viscosity of the chocolate will increase, which results in a very thick chocolate. If this happens, the good news is that you can save the chocolate without having to temper it all over again.
To fix the problem, you need a heat gun. With the heat gun, you gently heat the surface of the chocolate slightly, melting some of the form 5 cocoa butter crystals. Then you stir the chocolate, so that the heated top layer is thoroughly mixed with the rest of the chocolate. After a while, the consistency of the chocolate will become more liquid again.
It is possible to mould shells with over crystallised chocolate, but I certainly cannot recommend it. In addition to the fact that it is difficult to get fine, thin shells with an over crystallised chocolate, you also run the risk that the chocolate will contract less in the cavities. This is due to that a too large part of the chocolate has already crystallised before moulding.
Therefore, moulding with over crystallised chocolate will result in less shine, risk of dull spots, and less snap on the finished chocolate bonbons.
In my first e-book "CHOCOLATE BONBONS - a practical guide" you can read much more about chocolate tempering and how to solve typical issues in connection with chocolate tempering and the moulding of chocolate bonbons.