Don’t Use Tap Water
I have two irons - one that I only fill with distilled or filtered water, and another that I fill with tap water. 
I used to be even more meticulous about my clothes than I am now (a fact that will surprise friends). In college, I would only use distilled or filtered water with my iron. I kept the practice until my late-20s, but for some reason began to stray when I bought a new iron. So for the last three years or so, I’ve been filling up my second iron with tap water. 
This past weekend, I was ironing my shirts and noticed some brown spots on them. It took me a bit to figure out where the spots came from, so I ironed a clean dishcloth I have. Sure enough, the brown spots were coming from my iron. Apparently it spits slightly browned water now. I suspect there is some old mineral deposits in the spray valve or something. 
I switched over to my old iron, the one I used to only fill with distilled water, and it worked fine. 
I can’t say with absolutely certainty that my ironing malfunction was caused by tap water. However, I definitely won’t be chancing it again and will only use distilled or filtered water from now on. 

Don’t Use Tap Water

I have two irons - one that I only fill with distilled or filtered water, and another that I fill with tap water. 

I used to be even more meticulous about my clothes than I am now (a fact that will surprise friends). In college, I would only use distilled or filtered water with my iron. I kept the practice until my late-20s, but for some reason began to stray when I bought a new iron. So for the last three years or so, I’ve been filling up my second iron with tap water. 

This past weekend, I was ironing my shirts and noticed some brown spots on them. It took me a bit to figure out where the spots came from, so I ironed a clean dishcloth I have. Sure enough, the brown spots were coming from my iron. Apparently it spits slightly browned water now. I suspect there is some old mineral deposits in the spray valve or something. 

I switched over to my old iron, the one I used to only fill with distilled water, and it worked fine. 

I can’t say with absolutely certainty that my ironing malfunction was caused by tap water. However, I definitely won’t be chancing it again and will only use distilled or filtered water from now on.