Sequence problems during restart

We have a number of tables which have unique id's generated from a
sequence

i.e. create table blat (id serial, ....)

A number of these sequences seem to have been "set back" by 4 . In other
words we have records for

Id's 500, 501, 502, 503, and now the sequence  is set to 500.

The version of postgres is 7.0.3 running on RH 7.0.

We have recently restarted the databases, and upgraded our glibc.

Has anyone seen anything like this before?

TIA,

Dave