Heating p-Bromobenzenesulfonic acid in the presence of dilute HCl
I think @ron has hit the bull's eye in the comments! I would like to elaborate upon that: 



Since each step in the sulfonation of benzene is an equilbirum, sulfonation is a reversible reaction. According to Le-Chatlier's principle addition of $\ce{H+}$ ions in the solution would shift the equilibrium to the left in the last step of sulfonation where arenium is deprotonated, hence addition of $\ce{HCl(aq)}$ accompanied with heating for kinetic requirements results in the production of 4-bromobenzenesulfonic acid in the given reaction.  This process is called protodesulphonylation.