I had an underbilling experience when working at a small telecom. We billed based on 32 byte call record where each bit was significant. Some genius decided that since one bit's use was not documented due to a prior genius's laziness it was free game and he started using it. Unfortunately, the bit turned out to indicate calls that ran over 24 hours (generally computer to computer modem links). All of a sudden call revenue dropped and I was tasked with fixing it. The genius who had failed to document the use of the bit had left the company by then so I was denied any measure of revenge but the revenue loss would have paid my salary for years. It definitely prepared me for the Y2K kerfuffle later in my life.