Ugh that's the worst! Like @noah_webb said, it looks totally fine until you wiggle it. I swear half my "broken" stuff just had a sneaky break right at the connector. Always check the simple stuff first, saves so much time. Makes you feel kinda dumb when you find it though lol.

What a relief to fix it yourself and save that cash! Those little o-rings can be such a pain. My bathroom tap dripped for months because the new ring I put in swelled up and got weird. Is the one under your kitchen sink getting hit with hot water a lot? That heat really beats up the basic rubber ones. I switched to silicone for my bathroom and it's been solid for over a year now.

Honestly, I've never seen that kind of difference in my builds. I just use the paste that comes with the cooler and my temps are always fine. People probably just apply it wrong and blame the paste.