This one trips up new family editors as well as project modelers but I see it more often in the context of the family editor.
When you create a dimension string between three or more elements Revit displays the EQ icon/control/grip/toggle.
Clicking on this EQ icon will create a constraint between the elements that tells Revit that you want the spacing between each of them to remain equal at all times. This assumes that you intend to "flex" or change the distance between some of them or the overall distance at some point. Thus the reason I encounter it with family editors more often.
At the same time the Padlock icon/control/grip/toggle shows up underneath each dimension value.
You DO NOT need or want to click on the padlocks, which locks the dimension value. Doing so is similar to a "double negative", like, "I ain't not going to do that?"
Poor Revit is confused when you do this because you just told it to make sure the distance between each element should remain equal. Then you told Revit not to change them either! Adding a dimension and parameter to allow for flexing them makes matters worse! Up till now Revit tolerated your mistake...but trying to change the parameter will push Revit too far, it snaps and "yells" at you!
I suggest that we might be better off if Revit did not display the padlocks when the EQ constraint is invoked? In the meantime, just remember, double negatives ain't good English and it isn't not good for Revit neither.