Data corruption after an update set ...?

[Using postgresql-8.4.8-0squeeze1 under GNU-linux/debian]

Hi all!

I've just discovered that 456 records out of 1510 contained in a table
of my db bear the same content in two fields: the original data have
disappeared and I do not know how this occurred.

Luckily enough I'll be able to recover a large part of lost data but
would like to understand what happened: looking at .psql_history I can
see the command which might have caused the whole mess (see below)
although similar commands have been used in many other occasions without
doing any harm.

So the question are:
1. what caused the mess?
2. might depend on the processor (1300Mhz 10 years old)?
3. a HD failure?

I should be grateful for any hints.

Regards,
ennio

---------------------- .psql_history (partial)

_HiStOrY_V2_
..............
# update bibl_provv set titolo='Città scomparse. Tit.or.: «Versunken
# Städte. Eine Buch von Glanz und Untergang»', altre_notizie='Trad. di
# Enrico Massa. Prima edizione maggio 1971 nella collezione "I
# Garzanti"', n_pag_testo=363, car_tec_ed='br.fig.', largh_cm=11.0,
# alt_cm=17.8 where n_prog=10606;
..............
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