IMO its just one of those mistakes Optus can't really control well and since it means $2 to Optus every time it happens then they're not trying all that hard to properly fix it.
An Optus Rep will tell you that if it does occur all you need do is
1) First score your bills every day and notice it
2) Then spend 15 minutes - two hours contacting Optus
3) Run through the ID Process, explain the issue, get transferred to the right person
4) Wait for the rep to run checks, get told some data appears to have been used
5) Get told that Optus will kindly re-credit the amount
6) Check your bills next month to confirm the recredit.
Rinse and repeat for the next time. Its somewhat emblematic of that affable incompetence many Optus systems have built into them (Premium SMS subscriptions, Unlimited excess Data Fees, Roaming charges, Accessing account details etc. Front line staff are all happy to help but the amounts involved (i.e. $2) just make it not worth the occasional hassle for most customers (if they even notice)
FWIW It would be easy for Optus to fix this issue fairly. Currently if you use (or Optus think you used) 1 kB of your 500Mb allowance of data on the day you will be billed. Instead Optus could introduce a 0.2% tolerance. This would mean up to 1Mb of data could be somehow used before tripping the bills. That would handle these inadvertent data blips fairly and honourably.