Caught my own ground loop after 11 years in the field

So I'm doing a service call last Tuesday on a Vista 128 panel, and the customer's keypad keeps flickering and the transformer hums louder than a lawnmower. I started swapping out keypads and power supplies before finally deciding to meter the voltage at the panel. That's when I saw 14.2 VAC and realized I had installed this exact panel with the AC transformer and the siren power sharing the same grounding block. Took me 11 years to notice that little mistake, and it was causing all kinds of false alarms and low battery reports on three other jobs I did in the same month. Has anyone else had a similar "aha" where the problem was staring at you the whole time from a bad ground connection?

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