Another big mistake we have made in the past is not backing up the files we are changing. This takes an extra few minutes to create a save directory, copy the executables, and any other files you change. These files can later be restored if something goes wrong. This precaution is for those updates you are 110% sure (or possibly less) that the five-minute bug fix you just made will work flawlessly. Next thing you know the customer is calling asking why all the invoices no longer show the line items or worse , you have some how destroyed data.
We always feel most comfortable when the user has just backed up the data. If they do not have a current backup we ask they perform a backup before implementation. This is typically a scheduled event so there is plenty of time to ensure it is not only completed but is verified to be a good backup ( especially if done to a tape backup). With disk prices as cheap as they are these days we really prefer backups of this nature to be made to a second drive or another computer on the network. This provides the best performance in case you make a mistake and need to get the backup restored. The customers are never more anxious than when their system is down and the phones are ringing.