When the cashier lifts the mobile card reader in the sky to get a better reception you hear the sound of an online connection being established via modem. First, a modem requires a cable connection. And second, establishing a mobile connection via GSM does not create a sound.
The car Julius gets pulled over with at the border has wrong license plates (F ÄN 231). In Germany, umlauts are only used in the first part of license plates (which shows the city/county the vehicle is registered in).
When the group drives away from the border, they lose the front bumper with the license plate. They would have been stopped again by the customs agents or by any other police patrol as soon as possible.