I am using the EDD ticketing module and my site is set to NOT require user login for tickets.
In testing, I have noticed that if a user clicks the purchase button, taking them to PayPal, and then they cancel the transaction — either by clicking the cancel link on the paypal page or by using the backup button, the /checkout/ page then asks for wordpress login information:
To complete this payment, please login to your account.
Lost Password?
Along with a login form.
Our system would not allow purchasers to register, so that would be a dead-end.
Although this only pops up in the situation of the user backing out of an uncompleted tranaction, I can see that being a problem if the user wants to complete the transaction. (There could be valid reasons for an interruption that would lead to this situation).
Anyway, I wanted to bring it to your attention, as it is unexpected behavior and could cause lost sales in some cases.