orphaned RI constraints

I'm working with two different postgres installations - they're both
v7.1.2. On one I can drop a table and the related constraint info seems to
go away with the table, on the other the restraint trigger seems to remain
in the system tables and then when I try to delete rows from tables once
related to the dropped table I get errors saying the dropped tabled doesn't
exist - which I'm interpreting as the RI trigger trying to do its thing and
failing.

Questions:

1) Is this possible, or should I look for another explanation.

2) Can I fix things by dropping the constraint info from the system tables,
if so how? Is there a function which cleans the system tables checking for
problems like orphaned triggers, functions and sequences?

Thanks,

Andy.