nextval() on serial using old, existing value on insert?

[Wells Oliver <wells.oliver@xxxxxxxxx>]

I have a simple table with a given column defined like so:

 common_key    | integer |           | not null | nextval('alias.identity_common_key_seq'::regclass)  | plain

Very very very infrequently, on an INSERT where this column is not specified, this column will be assigned a value that already exists in the table, versus the next presumably unused value in the sequence. I cannot figure this out. Is there any reason why this might be the case?

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Wells Oliver
wells.oliver@xxxxxxxxx