Over Constrained Sketch error message when it isn't!

[oyaroygoldman]

I am getting the Over-constrained Sketch error message when I try to apply a dimension between a new line and an existing one, but there is no constraint applied to the new line except horizontal. What could be causing this?

I am attaching the file here - the line I am trying to define is the horizontal construction line in the top of the sketch. I am trying to apply a dimension between it and the first line below it. 

https://a360.co/41YCdXd

[TrippyLighting]

How would you determine hat this sketch isn't over constrained?

My recommendation would be to break this sketch into several smaller sketches and also avoid mirroring too many sketch objects.
 

[oyaroygoldman]

Thanks for the reply. 

That is what I ended up doing - I started a new sketch for that upper section, projecting some of the geometry I had already done. 

I am pretty new to Fusion and still figuring out when to start a new sketch. 

I was thinking the sketch wasn't over constrained because the line I was attempting to dimension wasn't constrained at all, beyond a "horizontal" constraint. I tried re-defining some of the lines I was trying to constrain the new line to, thinking perhaps the dimensions driving them were the issue. I even tried deleting and redrawing them with new dimensional constraints. When I went to redraw them, adding dimensions to them also resulted in an "over constrained sketch" error. 

Do you think the sketch mirroring I did is the problem? It does seem a bit messy to mirror mirrors like I did, but the lines I was dealing with weren't constrained to those mirrored sketch elements at all, as far as I could see. 

I guess I have the solution now, but I am still curious as to how these kinds of issues arise. Seems like every day a new problem emerges with doing something I swore I did exactly the same the day before. I guess that's learnin' for ya!

Cheers