Weird referencing problem

Has anyone ever had an issue with variables of type “movieClip” not correctly referencing their intended movie clip??

For example: in _root I have a movieClip called “chart1”. When coding inside the main time frame (_root), I will say x = chart1._x

and it will return “undefined” ONLY SOMETIMES.

However if I say _root.chart1 it always works.

What could keywords can I use to describe this bug? Am I only one that has come across this?