Different explain plans producing different results

I am posting this in the performance community because it appears a simple query that joins gl_balances with gl_code_combinations by code_combination_id is occasionally choosing a plan that produces incorrect results. I can print the code_combination_id from both tables and even though it is the join column the results show different code_combination_ids. This is causing a decode statement to be incorrect. Strangely enough if I simply rerun the query it uses a different plan and produces correct results. I do have an SR on this but was wondering if anybody has any ideas of what might be happening? My last thought