javascript - Magento Customer model rewrite causing seemingly unrelated JSON parse error -

warning, not faint of heart (sorry length)....we have implemented paypal transparent redirect custom payment method. long , short of have custom payment method takes credit card , submits paypal without credit card number ever hitting our server. takes number client side, sprinkle ajax, json , js, , voila(!), credit card approval , order submitted in magento. problem works great, except when custom module rewrites mage_customer active. have narrowed 1 model have overwritten. if comment out rewrite node in config model, transparent redirect works fine. have determined not code within models, have removed 100% of it. declaration remains :

 <?php    class mycompany_customer_model_customer extends mage_customer_model_customer  {  } 

this alone somehow causing json parse error when request sent paypal. there having child class part of process break javascript? not experienced javascript or json, know response coming back, , looks identical success response, yet somehow "onexception" being triggered. message: syntaxerror: json.parse: unexpected character.

i don't see how 2 related. although have had other strange issues related model overrides in past. example, did rewrite on customer create api add additional attribute, , when incoming xml hits custom method, object (not array), workaround forcibly convert array, however, when arrives in standard method, array start (i not need this, works).

i figure there must explanation out there of these strange issues. if has advice on how troubleshoot this, or information/experience why inherited classes can break things, appreciated.

we haven't solved yet, we're on our way. we're developing on windows stack , manually saved customer model utf-8 encoding. however, partner developed of paypal extension , did not save in utf-8. when converts model ansi format, code works, he's going try other way around (making utf-8) , checking it. if has advice on encodings , development platforms, i'd hear it, still.