Displaying the DRC error during slide via into copper shape.

When using the slide command  and a DRC error is displayed, 2 triangles point at each other , how do you tell what the DRC is ?

I get this error when moving a stacked via into a copper shape(static solid) of the same net name. This occurs on one board I am working on but not on another. Why would you get an error when sliding a same net via into a copper shape ?