SERIAL error

I'm using v. 7.4.2, and am encountering a problem with serial types.  On an
insert into a table that results in failure for any number of reasone (mostly
violation of contraints) I've found that the Sequence for the column of type
serial will still bump the value up once.  This can be seen when entering the
next value with no problems, the value is 2 greater than the last value.
Noticed this when experimenting with a serial column with a unique constriant,
and then manually raising the serial value of a row to the next number due in
the sequence, the first attempt caused an error, but on a second attempt it
worked fine but the number was one higher than what i expected.  It also occurs
for errors pertaining to other columns, when using a two column table, one
column with a not null constraint, and the other the serial column, entering in
a null value for the first column causes the sequence to fire, and then
following that with a valid entry you'll see the value for the serial column is
+2 of the last value.  This is still the case in a transaction.  Anyway, was
wondering if this needed to be reported as a bug?  Seems to cause problems for
certain generic uses for the serial type.

Matthew Story