Prompts with wrong parameter name to enter for variable. (4242251)

I have a very peculiar problem. A few days after upgrading from 9.7.2.5 to 10.0 (with 10.1 patch), I started getting prompts for the wrong parameter name. These incorrect prompts happen infrequently, but consistently.

e.g.
--12345678901234567890
SELECT * FROM DUAL
WHERE DUMMY=:DUMMY1;

In this SQL, I get a prompt for :CUSTID (a common column in my database). However, removing or adding a single character on the first line causes the correct :DUMMY1 prompt to be given.

When the incorrect variable name is given, the SQL does not run and no error occurs. The SQL works correctly with the correct variable name.