I have a template with the parameter "position" and I'm also able to create a custom property "position" but the reference to both of these properties in an expression is the same, so I have no idea which one it's going to grab. This should probably not be allowed.
**Edit: I think this was a side effect of adding a parameter to a template, an instance of which already had a custom property with the same name. I cannot add a custom prop with the same name if the parameter already exists. I doubt that there's anything that IA can do about that situation because of the order that things were done. So, more of a PSA.