I'm sure this happens often ... well, at least that's my excuse for having done similar with an AA booking - I only became aware of my mistake when I couldn't access the booking and phoned - the agent didn't seem that worried and putting in my first name instead of my surname meant I could get to the booking.

One slight problem was my BA Tier points didn't post and I had to send in booking and boarding card, so hang on to the paperwork.