If some sperm accidentally dripped down from her lower back through her"butt crack"and got in her vagina opening what are the chances of pregnancy?

Small: But anything is possible-please remember to use condoms to protect against stds.

Small but possible: It just takes one, right :) actually, that is too simplistic but if someone really does not want to be pregnant then a good contraceptive is necessary to avoid these "accidents". Be careful as contraception is usually not that expensive and it is very effective (pill and iud). Best wishes.

Very doubtful: Unless you have outstanding swimmers, the probability of ejaculating outside the vaginal opening, and a resultant pregnancy are minimal to zero. Try a home pregnancy test to be absolutely sure, but my professional opinion is if you want to get someone pregnant, try ejaculating in the vaginal vault. Otherwise, your just getting some exercise, and burning some calories, and hopefully having fun.