It is quite common for the hinge pin on a tilt head mixer to work loose.  When it does, the pin will be uneven on each side of the base.  When mixing the mixer will often "lurch" from side to side and the beater may strike the sides of the bowl.  Fixing the problem is quite simple:

  1. Unplug the mixer
  2. Using a rubber mallet gently tap the hinge pin until it is flush with the side of the mixer.
  3. Using a dish cloth to protect the head, turn the mixer upside down and hold still.
  4. Tighten the flat head screw to secure the hinge pin