Bingo. When I worked in swimming pool supplies, we were warned never to mix pool chlorine with pool acid because it would create mustard gas. Phosgene has a similar composition. Both mustard gas and phosgene were used in WWI.
Reading up a bit on what it takes to produce mustard gas, I find no reference to combining pool acid (normally hydrochloric acid) and pool chlorine (normally a solid compound of chlorine).
It would, most likely, "just" release chlorine gas, which was also used as a chemical weapon in The Great War, and thus also undesirable.
Mixing pool chlorine and acid will yield chlorine gas, not mustard. Mustard and phosgene are entirely different compounds. At room temperature, mustard is a liquid, phosgene is a vapor.
Supposedly mustard gas is synthesized from ethelyene glycol and dry hydrogen chloride, but you won’t get it from pool chemicals - Chlorine maybe. That would be bad enough.