I once rented a bedsit which had a distinctly dodgy meter, if you allowed it to run down to no credit the lights would flash briefly and the little needle would start a new trip round the dial. I discovered this by accident but of course bring an honest type I always used to top it up with 50ps. Actually it was more out of fear that the damned thing would break and I would be held responsible for whatever shortfall the previous tenant had built up.