Using "Go Back" on Paypal Express breaks contribution form

Steps to produce:

1. Create payment processors for Paypal Express, Authorize.net, and Google Checkout. (I didn't create full working ones – just enough to play around with the multi-payment processor UI.)
2. Create a contribution page which allows use of any of the payment processors
3. Open the contribution page
4. Select "Paypal" and click "Confirm Contribution"
5. On the next page, click "Go back"
6. Observe: There are no visible buttons for submitting the form

The only way to get the buttons again is to restart the contribution process (/civicrm/contribute/transact?reset=1&id=X).

This was observed under branches/v4.2 (r41670) with Safari 5.1.7 and with Firefox 14 (OS X).