I could be wrong, but I think there is a conflict when the payment modules of Braintree and Paypal Express Checkout are used AND a customer goes through the checkout path created by the Paypal Express Checkout button (customer details pulled from Paypal rather than the Zen-Cart site).

For a long time I have used the payment processeor Paypal Express Checkout (for Paypal) and Paypal Payments Pro (for CC processing).

I recently removed Paypal Payments Pro and activated Braintree instead.

I then had instances (maybe once a day) where orders had:
- the correct ship to details
- no bill to details
- customer details that didn't match the ship to and instead matched an existing different customer
- Paypal invoice/transaction ID's assigned which matched other transactions
- no actual charge in Paypal (so it wasn't just that the transaction ID's were incorrectly recorded in the Zen-Cart database)

The only fix was to remove the Paypal Express Checkout button.