I'm sure it's been pointed out before; however, with hundreds of threads...
Here's a lesson in proper chemical addition.
The compound "acid MAGIC" cited as a sanitizer in the article is a muriatic acid replacement product. If the people running the pool believed it was a sanitizer and added it at the same time as they added the chlorine product to the pool... IMHO: Sounds to me that someone tried to shortcut and added both chlorine and acid to the pool at the same time and was using an in-line mixer!
So just in-case: It is important that when adding the chemicals to the pool water that they be allowed to mix in the pool for awhile before adding the next product... and never, ever, add both chlorine (bleach, liquid chlorine, etc...) and acid at the exact same time!