Could be your stars weren't properly aligned that day.

But there is a lot you aren't telling us. What exact Cinelli Mash frame? What exact model and series BB and crankset?

What has the person that put it together told you? Certainly you showed them that it is loose and ask if they could do anything about it.

Is it just a crank arm that is loose? You might be screwed if you rode it too long with a loose crank arm.