pdirect: It's not a custom screen
I convereted a lead to a customer through the opportunity screens in two ways and neither produced an error so please let me know what you did different from me. From the Leads and Contacts List screen I clicked Go To... on a lead then clicked on the Opportunities section tab. Here I clicked New Opportunity... filled out the required information and left my stage at 50%, click Save & Close. Next I clicked on the Opportunities tab and clicked Go To... on my newly created opportunity. On the Edit Opportunity screen I changed the stage to Closed Won and then clicked Save. While saving I get a popup asking if I'd like to convert this lead to a customer so I OK it and the page saves fine.
I tried this several times with different leads/opportunities and was not able to cause an error message. If you can, please give a little more detail about which screens/steps/information you're using on your end and I'll recreate the scenario as best as possible.
~C