We usually don't use a rental car when in Europe. We typically have a private car or use public transportation but this summer, since we were traveling in Ireland, we rented a car.
Upon returning we received a notice from our rental company stating that we hadn't paid one of our two tolls on the M50 leaving and returning to the Dublin airport. They charged us for the "unpaid toll" and a small administrative fee. We had paid both tolls (prepaid the second) within the prescribed time after leaving the airport. We have an email confirming both payments but we were still charged by eFlow because they used one of our payments to pay for the people who returned the car to the airport the day before us! After a call to eFlow in Ireland and Enterprise in the U.K. we are told our charge will get reversed.
Just an FYI that even if you do everything correctly sometimes things get complicated. Stick with it. Keep your documentation. Make those international calls (it took two Skype calls) to get things resolved. Enterprise says they will issue credit to our cc for the charges that were assigned to us on the day before the rental car was ours. I wonder if the previous renter will now get a bill for the day s/he returned the car before we picked up the same car? Ugh, I really don't like to rent a car in Europe but I would agree that it is so beneficial in Ireland if you want to do things at your own pace.