As reported by the German newspaper Die Welt among others, Ryanair seems to be under authority investigation for having declared a MTOW of 66.990 kg as basis for the calculation of air navigation and landing fees but having systematically exceeded this. Every flight examined by the authority has had an actual takeoff weight above this value, but equal to or below the Boeing-issued maximum TOW of 74.990kg.
It is expected that within Germany alone, the savings amount to around 370.000 Eur. a month. The immediate future seems to hold a sizeable bill for Ryanair.