I think it depends on what is deemed by term "love", since it can mean so many different things to different people. I think it's posible to love someone at first sight based on just a very limited understanding of the person, their personality and frame of reference. Sometimes its instinct, sometimes its acts of selflessness that the person does right there and then. But once again, word "love" will mean something different to most people.
Personally, an experience I had with an ex can be desrcibed as love at first sight. It lasted for awhile until it got extinguished by the winds of change.
Don't cry, don't regret and don't blame
Weak find the whip, willing find freedom
Towards the sun, carry your name
In warm hands you are given
Ask the wind for the way
Uncertainty's gone, your path will unravel
Accept all as it is and do not blame
God or the Devil
~Born to Live - Mavrik~