Suppose there is a table "mytable" with column col_pk as follows.

col_pk
----
1
2
3
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And update and insert sql as follows.

update mytable set col_pk=4 where col_pk=3;
insert into mytable (col_pk) values (3);
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If you run the sql above only once, it never yields error. But if you run the sql twice, it will yield error because there is already 4 in mytable.

I'm not sure. Just guessing.