HCl + NaClO = Toxic Gas

Repeat After Me: "Mixing hydrochloric acid (aka pool chlorine) and bleach is B A D".
This will always result in a nasty chemical reaction that releases heat, and toxic vapors.

Yep, it happens every day. Except this happened on the grounds of a major Denver-area hospital. Read the article here.

Note: This hospital specializes (ironically enough) in respiratory diseases.