If the HT (high tension/voltage) is adjusted above 142 volts (in order to increase brightness), internal parasitic capacitance between the tube's anodes can produce some digit "ghosting". This capacitance is there because of a) the tubes internal design, and b) the multiplexed display design.
In order not to damage the tubes, or reduce their lifetime, its better to reduce the HT to 142 volts or until the ghosting goes away. But if there is still some ghosting at the desired brightness, Frank Bemelman suggests the following modification, which will drain any residual charge inside the Nixie, and solve the problem. The overall display looks generally sharper after this modification.
You will need 11x 1N4148 or 1N914 diodes and 2x 47KR 1/4 watt resistors. These parts are mounted on the PCB underside in existing holes as per the diagram below.
Mount the two 47K resistors in the red holes, and the diodes in the green holes - the anode end goes into the hole. Bend all the component ends and solder them neatly together. Keep everything 'low-profile' so that there is no possibility of a short circuit with the steel case. Further details can be found here.