Problem while reading from some tables

If a make a very simple query from the psql shell:

nuovo_mercato=> SELECT * FROM prezzi;
pqReadData() -- backend closed the channel unexpectedly.
This probably means the backend terminated abnormally
before or while processing the request.
We have lost the connection to the backend, so further
processing is impossible. Terminating.

..and psql dies leaving me a bash.
My database "nuovo_mercato" contains 5 tables. 3 of them are
accessible, 2 are not. Sometimes I read something like this:

nuovo_mercato=> SELECT * FROM var_ieri;
ERROR: var_0ieri: Table does not exist.

but I wrote "var_ieri" and not "var_0ieri"!! Of course table
"var_ieri" *does* exist!

What's happening?
I use postgresql 6.5.3-26 on debian GNU/linux potato

Thank U