'invalid' value in timestamp with timezone.

Hi all,

I've noticed for the first time in some 7.1.2 databases that
I use, that somebody/something managed to insert a literal
alpha value of 'invalid' into 'timestamp with timezone' columns.

I tested it myself, and it worked :
test712=# create table bob (field timestamp);
CREATE
test712=# \d bob                   Table "bob"Attribute |           Type           | Modifier 
-----------+--------------------------+----------field     | timestamp with time zone | 

test712=# insert into bob values('invalid');
INSERT 3885934 1

On 7.3.4 it gives me :
test734=> insert into bob2 values('invalid');
ERROR:  TIMESTAMP WITH TIME ZONE 'invalid' no longer supported

but other strings gave me the same error on both db's :
test734=> insert into bob2 values('valid');
ERROR:  Bad timestamp external representation 'valid'

I looked in the documentation and on the net, but cannot find
much reference to this "ex-feature"

I'm 80% sure no one actually inserted these 'invalid' values into tables,
and what I need to know is : 
What else can insert a value of 'invalid' into a 7.1.2 
"timestamp with timezone" type column.

Was there a function or something similar?

Regards 
Stef