1) After the fitting of a set of solar panels the consumption meter was not upgraded. The meter that was in place was one of the older rotating disk types. As the solar generated power was exported to the grid the meter unwound the previously recorded consumption. The householder was paid for the power generated via a generation meter and received a discount as the previous consumption was unwound. That is double accounting. For one summer quarter the total electric bill was 1% of the normal/expected amount. The situation was unwound when during another summer the consumption bill reading was negative and the electricity company realised the mistake.
2) After moving house into a slightly larger house, it was found to have a dual rate meter. These meters known as Economy 7 are matched with a special tariff arrangement where by the cost of electrify at night is cheaper than the day rate. The day rate is higher than than the normal day rate. The meter shows three numbers in sequence labeled 1,2,T The 1 & 2 numbers are the two rates and the T number is a total of the two rate numbers. After a visit from a real meter man a letter was received from the utility company sating that the number types from the meter had been entered round the wrong way and would be corrected in future. Turns out that the day and night number had been reversed and the house had benefited as a result of that. Like most houses we use more electric in the daytime despite my (now futile) efforts to put the dish and washing machines on at night. There are solar panels to cover some of the daytime consumption and under floor heating to eat that up.
That was two metering errors that worked out in the houses favour, most certainly an unusual sequence of events.