"attempted to lock invisible tuple" error while update

Hi All

I am using a cluster setup with two nodes in it. Replication between
two nodes is being done through slony.

Postgres version is 8.1.2 and slony version is 1.1.5 .

I am running an operation that does thousands of update/inserts/delete
on some tables. While running update query on a

particular table let say <abc> , it gives error "attempted to lock
invisible tuple" and fails. This update query is supposed to update

hundreds of record. Sometimes it gives the error while running this
update query and sometimes not. I have seen postgres logs of two

instances when it fails due to "attempted to lock invisible tuple"
error , in these two instances while it was doing an update on <abc>
table,

another query was going on the same table which was updating a
particular record. Can this (two updates at the same time on the same
table) be the reason for

this error in some way ( just a guess) . If not, then what can be the
reason of this error ??

Please help.

Thanks..

Tamanna