We rented a car from Sixt while our family was on vacation in Spain last month.  Today we received a fine for 
NO RESPETAR LAS RESTRICCIONES DE
CIRCULACIÓN DERIVADAS DE LA ZBE DE ESPECIAL
PROTECCIÓN DISTRITO CENTRO
This driving rule is unique to Spain and should have been explained BEFORE renting the car.  The Sixt agents knew we were foreign because of our driver's license. They also knew we would be driving through the restricted area, which is apparently the entire tourist area in Madrid.  It is unethical not to explain this rule.

Sixt then charged us 15 euros for being ticketed, and an additional 24.20 euros to assist in paying the 100-euro ticket. 

We also returned the car a few hours before it was due, but the person didn't record the return properly. We then spent hours on the phone, during our vacation, getting it resolved because they were trying to charge late fees.