Can abortion fail?

... thing first, before delving into the homemade remedies, but I need it to fail.  But, I need that failure to be plausible. Any way to make it work? I've tried posting this in Childfree and AbortionInfo, and got sent here.  I've checked google, which states that certain types of abortion (I'm not too clear on the differences at this point) have a 96-99% success rate, which means they do sometimes...