Has anyone else had issues with Mac users crashing during checkout?

We've had 3 or 4 over the past couple of months.  Most of the time the order goes through, but the user says the website crashes.  They get a confirmation email, but not the confirmation page.

It appears this only happens with Mac users.  We are on 4.5bto with no customizations done to the OnePageCheckout system.

We're working on upgrading to 5.0, so maybe that will fix the problem, just wanted to know if anyone had any ideas, or if anyone else has experienced the same thing.

Thanks!