Capital one accept online then change their mind

I applied for a credit card with capital one and was told online that I had been accepted and I signed the credit agreement online. I got nothing in the post so called them and was put on hold for a while after telling him my details. He then came back on the phone and said a manager would call me tomorrow. I asked why this was happening and he was reluctant to say that due to a technical error I had been told I was accepted but I had actually been declined.
What I am wondering is if the credit agreement for 6 months 0% balance transfer can be cancelled if they wish or if it is binding from both sides?
Thank you in advance