I was getting PayPal standard payments set up for our new website. We already have it running on other websites.
I put in a test order using our PayPal/PayFlow email address as the email address.
When I got to PayPal I had to change the email address because, of course, I was paying for it with our selling email address.
It completed the order and took the payment but never recorded the order in ShopSite.
So, are you aware that changing the email address while in PayPal does not complete the order on ShopSite but PayPal does complete the payment?
I can foresee users doing this when entering the email address they use the most on ShopSite but then entering the email account that they use for PayPal on the PayPal website. Probably doesn't happen that much, but eBay users often set up different email addresses to keep the riff-raff out of their most used mailbox.
I only tried this once so I didn't reproduce it again.