We're used to requiring a match in order to make something explode. No need this time. When these two gases meet, they explode on contact. We'll tell you why. (And show you how cool it looks.)
Chlorine was never a good gas to be around. It does its work by yanking apart other molecules. During wars, it was used to shred peoples' lungs. Today, it's much more likely to take apart any biological stuff that crawls into swimming pools. Most of the time, it's only the besieged molecule itself that has a problem with chlorine. Occasionally, when chlorine sets to work, it takes out not just the molecule, but everything around it.
This is the case with acetylene gas. Acetylene is also not a gas to be played with, as it supplies the fuel for welding torches. It generally needs a flame, or excessive heat nearby, to ignite. If chlorine is around, it ignites without any further help. Acetylene is pretty simple - it has two hydrogen atoms and two carbon atoms. Those two carbon atoms are bound together in what's called a triple bond. Six electrons are all invested in keeping the two carbon atoms together. That's a lot of energy, and when chlorine comes in and starts ripping the atoms apart - kaboom.
There are lot of demonstrations online of acetylene and chlorine putting on a show. They even manage as bubbles underwater. So, yes, you should be alarmed by a leak of either gas, but the two together? Get running.