You may receive the following error after installing the PayPal Pro payments contribution module from OsCommerce:-
"Paypal oscommerce Authentication/Authorization Failed (10002)"
Referring to your username and password. If you've entered your actual API username and password (not your PayPal username password - the two are very different things!) then the next thing to check is the certificate.
Typically, when you download your API certificate from PayPal it may add a .txt extension, remove this and replace with a .pem extension, update the configuration in the admin to relate to this new file and bingo - payments processed.