Mixing hydrochloric acid and sulphuric acid, both concentrated
Three things occurred. First, sulfuric acid as an high affinity for water (it is indeed used as a drying agent in some cases. For instance, if you wanted pure and anhydrous bromine, you could bubble wet bromine gas in concentrated sulfuric acid). So, one thing is the "stealing" of water by sulfuric acid, which took the solution one step forward the saturation point (with the release of gas).

The release of gas was aided by the exothermic reaction of sulfuric acid with water: (self) heating of the solution helped releasing the gas.

Edit: the following is an hypothesis, made by assuming that the "extreme" reaction conditions could have aided an unlikely redox reaction, but I am not sure about the oxidation of HCl:

And finally, sulfuric acid is a oxidizing acid: it oxidized part of the hydrochloric acid to chlorine, which made the solution green (which probably was released as a gas, together with HCl).