System Information
Windows 7 64, HD 5770

Blender Version
Broken: 9337574 (2.71 release) & 1987548

Short description of error
Instanced objects (mesh or curve for example) are going crazy (spin around)
if minimum 2 in the same instance have a Cast modifier, same target object, Use Transform and Edit Cache activated.

Exact steps for others to reproduce the error

• Add an Empty.
• Add a Cube, give it a Cast modifier.
• In Cast modifier:
  • Set control object to Empty.
  • Activate Use Transform option.
  • Activate Apply modifier to editing cache during edit mode in the header.
• Make a linked duplicate (ALT+D) of Cube.
• You can place all objects how you like.

You have now 2 linked Cubes, using the same target for Cast.