"there may be sperms present in the 'dew drop' of fluid that a man produces when he is excited, but well before he actually ejaculates."
http://www.netdoctor.co.uk/sex_relationships/f acts/coitusinterruptus.htm
"The withdrawal method of birth control has an average failure rate of 19%. Although it can be used effectively, it is not uncommon for an unplanned pregnancy to occur when a couple uses this type of contraception. Moreover, it is important to note that men have what is known as "pre-ejaculatory fluid" which seeps out from the penis when a man has an erection. While this is mainly made up of lubricating fluid, it is possible for a few sperm to be present in this fluid along with certain sexually transmitted diseases, including HIV. Even if a man pulls out during sex, there is still a possibility that pregnancy may occur. "
http://www.epigee.org/guide/withdrawal.html
For couples that use coitus interruptus correctly at every act of intercourse, the failure rate is 4% per year
http://en.wikipedia.org/wiki/Coitus_interruptu s