A few of our clients are using  PayPal Pro Hosted as a way to collect and process credit cards on their Magento sites.  Fortunately, it is the least used of the Paypal options built into Magento by default.  We have been helping a few clients track down an issue whereby a number of customers were unable to pay at checkout.  In the case of 1 client about 20% of all orders were failing. With any payment gateway issues there are a lot of reasons why something can go wrong. After painstakingly eliminating each possibility server side we were left with the conclusion that it must be client side.  So we went back to the Apache / Nginx log files and found every instance whereby a client failed to pay and analysed the USER AGENT of the browser.  It was quickly evident that Safari was to blame.. not just on iPad or iPhone but even Safari running on Windows. In all cases the embedded iFrame failed to show into which the users credit card details should go.

Once this was discovered we turned our attention back to PayPal.. who have known about this for months:Paypal forums

So if you are using PayPal Pro Hosted and you are seeing a strange number of Abandoned carts after the user has Placed their order there is a good chance those users are on Safari.  Depending on what you are selling this could be a significant percentage of your customers.