The most common way, that causes problems, is when pool owners have chlorine tabs in the skimmer basket, and pour bleach or add cal hypo through that same basket.
Some years back, a PoolForum user reported doing this. He turned his back and started to walk away, only to here a KAPOW! When he turned back, his neighbor was pointing up in the air. When he looked up, he saw a small white skimmer lid that was beginning to fall back! Or at least, that's how I recall it. I believe that post may have been before 2006, so it would have been lost in the forum crash I had.
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