Because they always end up leaking! Reminds me of the time I was called to fix a joint that was supposed to be leak-free for five years. Turns out, nothing is truly sustainable when it comes to bad workmanship. Anyone else have funny stories from the field?
Gotta love those ‘leak-proof’ guarantees! Had a similar joint that turned out to be more drama than I expected. Ever had a meltdown mid-fix? 
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Funny how you mention a leak-free joint lasting just five years. I once worked on a copper fitting that seemed to hold up until winter came, and then — boom! Frozen pipes revealed the flaws.