BUG #3991: pgsql function sum()

The following bug has been logged online:

Bug reference: 3991
Logged by: mirek
Email address: mirek(at)mascort(dot)com(dot)pl
PostgreSQL version: 8.2.4
Operating system: Linux fedora(for work) and windows(for development)
Description: pgsql function sum()
Details:

I saw a very strange behavior when i used function SUM() in query (used
inside pg function), query:

select magazyn, ciag, numer, datawp, vat, sum(wnetto-wnettop) as wn,
sum(wvat-wvatp) as wv, sum(wbrutto-wbruttop) as wb from vzk_pozycjezinfo
where magazyn = $1 and ciag = $2 and numer = $3 and datawp = $4
group by magazyn, ciag, numer, datawp, vat
order by magazyn, ciag, numer, datawp, vat

vzk_pozycjezinfo is view

problem is in: sum(wvat-wvatp) where field in view wvat = 33.08 and wvatp =
36.09
Result is -3.01000000000001

If I ask postgres manualy: select sum(33.08 - 36.09)
result is ok -3.01

Now I fix it with round function but i think that is a bug.

With regards
Mirek