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A1 - Check to see that your Comp Settings and your Solid settings have matching pixel aspect ratios. Look at the "Composition Settings", if your pixel aspect ratio is set to "Square Pixels", make sure the "Solid Settings" for the layer that has Reflector applied to, is also set to "Square Pixels". If the comp is set to 0.9, then the reflective layer also needs to be 0.9.
A2 -Make sure you are using the Comp camera for everything. Reflector uses the comp camera by default, but you'll have to turn it on for the 3D Invigorator and ProAnimator plug-ins.
A3 - If you are working with 3D objects (ProAnimator/Invigorator objects for example), make sure you set the Color Solid to the Comp size.