I made a number of bookings in Europe with booking.com including one hotel in Rothenberg. The GASTEHAUS VIKTORIA which charged me much more than what was on their own website.(do not stay there unless you like cramped little places)
When I got home, despite the fact that I could not give BOOKING.COM a printout of the lower rate, they still honoured the Guarantee and refunded me the difference in price.
This goes beyond the call of duty and Booking.com Customer Service were great to honour this.!