When adding a new event registration (from contact/view - Events tab)....
If you leave a required field empty (e.g. Participant Role) - AND check "Record Payment" and fill in the Payment Information fields (e.g. Paid By, Contribution Type etc.) - these values are lost when the form reloads with the formRule error. (and the "Record Payment" checkbox state is not retained)
This can cause user errors (as it did for me) - since the user will correct the error but not notice that the other data they've entered is not longer present.