Roughly, constant boiling HCl is 18% while conc HCl is 37% (I don't remember the actual values). When you reflux, there isn't enough water to absorb the HCl as the higher temperature.
There had been an earlier post in which someone advocated refluxing an ester in conc HCl. At the time, I thought about making a note about the HCl pouring out if you do this, but since the concept was basically correct, it would hydrolyze the ester, I let it go. Here is why you don't want to just do this. If you want to reflux HCl, dilute it to constant boiling HCl (~6N) first.