A un-initialised varible made our script to mail 20 copies of the same email to almost 10k users. One of our mass email script basically breaks the email list i

A un-initialised varible made our script to mail 20 copies of the same email to almost 10k users. One of our mass email script basically breaks the email list in chunks of 500 and emails them. The array variable that hold this 500 email chunk was not initialised. In our test everything was fine as the test list was never more than 500. When the time came to send emails to the production list, the script basically kept sending multiple copies of the same emails (since the array retained the old values across the loop iterations). I realised this after about 10 minutes but already multiple copies of the same emails (20/user) was send to more than 10k users!...The end result was annoyed users unsubscribing in droves and possibly a hit in our email reputation. This is all because a stupid array variable was not initialised :)