System: Win7 64x
1.Create a sprite that is immovable and place in middle of the screen.
2.Spawn another sprite on top.
3.Exception generated at line 1017 in sprite plug-in.
Include a capx and detail further please, I can't reproduce the issue.
The fact that you say "Create a sprite that is immovable" let me thinks that you're adding a physics behavior to the sprite, but it's not clear.
Also I guess the step 3 happens when you preview your project, not directly in the editor. Once more, unclear step.
Really do provide a capx and put more detailed steps should help.
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Please include your .capx.