When creating an aggregate, if the used entities have attributes with the same name as local variables, these attributes currently silently hide the variable, and the user is not able to select it for the aggregate (e.g.: in a filter).
Instead of silently hiding it, I suggest displaying the variable where it normally would be shown in the expression editor (under the "Local variables"), grayed-out, with a mouse-over popup that explains briefly why isn't that local variable available (e.g.: "There is an attribute with the same name as this variable. Please rename the variable to use it.").
Something similar should happen for auto-complete, for power users that type rather than select variables. For example, the only auto-complete option could be the attribute, but a red text line under it explains that this is the attribute rather than the variable due to name collision.