Unforeseen challenge. Suggests a problem suddenly occurs that was never considered. In baseball, a curve ball is most difficult pitch to hit against, thusly anything that throws a curve ball is hard to handle.  (Compare to Tear-jerker, Wrench in the machinery,  Life’s little test or Sob story.)

“When she said she would accept the job, but wanted to use her vacation immediately, it threw  a curve ball into the decision.”