What causes a table's oid to change?

I have a table with many views. I did something to cause the table to change 
oid. I don't know what I did. When I try to access the data using one of 
these views I get "ERROR:  Relation "documents" with OID 24389201 no longer 
exists". The table is fine. If I drop the view and recreate it it works fine. 
Can you tell me some of the common thing that would cause this? Can I "reset" 
the oid?

Thanks
Richard