UPDATE with JOIN subtle bug

I have been diagnosing very subtle bug in SQL code which led to unexpected results. It happens under rare circumstances, when you do update with join and you want to increase some number by one. You just write value = value + 1. The thing is, you are willing to increase the value by the number of joined rows. The SQL code kind of expresses your intent. However, what actually happens is, the existing value is read only once. It is updated 3 times, indeed. But with the same value, incremented only by one.

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