We're currently having a weird bug on our magento site. Sometimes a customer send us an email complaining about the shopping cart. The subtotal is indeed calculated wrong.
See an example here : http://imgur.com/ZDtgm9r
Our team was unable to reproduce it by any mean. Today we have the case where a SINGLE, simple product got its total mult by 3...
I dont know how i can help you, which file you may need or so. Neither totals.phtml nor Totals.php (where renderTotals() is defined) have been overriden.
This is probably close to this question : Why Magento Shopping Cart show wrong price and sometime not allow to add product?
We're stuck with it for weeks now... And we have no clue since we're unable to reproduce it. So here's my call to community. Did you have this bug ? How did you resolve it ? Or maybe do you have an advice from where it may come from ?
EDIT : If a customer checkout the whole cart, all the prices during checkout are OK.
Thanks to all of you