I can report the same problem on Oracle 8I with the latest PL/SQL Developer.
I upgraded tuesday, and have not seen this before. On a Oracle 8i system I get the same error.
To reproduce:
Restart PL SQL Developer
Logon to Oracle *i
I execute a query.
Click in a field in the result grid.
Then the error only occurs once after restarting the PL SQL Developer.
Hope to have been of help