What will happen if someone can't find a reference for teaching when he applies for an academic job? This may happen if someone has left academia but want to return after a couple years. Thanks!
If you have no teaching experience at any academic institution, it's obviously going to be hard to get such a reference. Note that people do sometimes move into academia having had no prior academic experience, but it's not common. I think in general your application would have to look fairly compelling, in which case the absence of a teaching recommendation may not matter so much.
On the other hand, if you have had teaching experience at an academic institution, then there are probably records of it there (e.g. course evaluations). In this case, you can ask an appropriate person from that department (a calculus coordinator, director of undergrad studies, or someone similar) to write a letter for you based on those old evaluations.