Plumbing Questions? Ask a Plumbing Expert.
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You didn't miss anything except that perhaps you need a thermal expansion tank for the water heater or the tank you have has failed. What likely happened is that you filled the tank entirely with cold water and when that water heated up the expansion of the cold water raised the water pressure to the point where your relief valve discharged. It may never happen again but you'll need to replace the relief valve if it doesn't stop leaking.
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