I've done this before plenty of times and once even brought my balance to $0.01. No problems.
The next morning, I look at my phone and I see an email from Wells Fargo. An overdraft transfer? wtf? Yes, they transferred $35 from my savings account to my checking and charged me $10.
Annoyed, I left home early before work to make that dreaded stop at the bank. I explain my issue to the banker, who can't figure out this magical overdraft either. She goes and talks to the manager about it, and he tells her that their computer system will actually analyze your purchasing history and predict that you will overdraft. HAHAHHA. WHAT.
So she tells me "It was a computer error" and refunds me the $10. This is great and I am happy about that and I think she gave me good service. But what the fuck? How is this an error if your system is programmed to do it? What's worse is I am pretty damn sure this has happened to me before when I paid less attention to my account. :S What the hell, Wells Fargo?
TL;DR: Bank does an overdraft transfer and charges me $10 for that "service" when I did not overdraft. Says their system is programmed to predict when I will overdraft and preemptively mess with my money.