I was trying to do some stuff with CoreAnimation and ended up with this compile error:

Undefined symbols for architecture i386:
  "_OBJC_CLASS_$_CAKeyframeAnimation", referenced from:
      objc-class-ref in SpellPageViewController.o
ld: symbol(s) not found for architecture i386
collect2: ld returned 1 exit status

To fix it: Make sure you have linked the QuartzCore library.