It's happened to me a few times with the 50MB. But as it was only small amounts I just let it go. Last month I purchased a 1GB data pack for my brother for him to use while he was in hospital. I topped up today with $20 and to my surprise I was instantly auto renewed with a 1GB data pack even though I had stopped that service months ago. That wiped out all my credit.
Since I would never use anywhere near that much data in a month, I rang customer service asking for a refund. I was basically told that they no longer refund.
I don't understand why they're not willing to as I still want to use the credit, just not all my credit for data.