Does anyone have any idea why this happens?
Brand new install of Blender 2.79b, very first model I started. Snapping quit working and then I find a hole in the mirror.
I can't really see what happen with your picture.
The mirror modifier use the origin point to create a symetry of your model according to the axis you choose.
So... be sure your origin is put at the right place and Apply any rotation/scale you could have done with CTRL+A.
Maybe it can help, otherwise don't hesit to put another picture of your scene to see what is goin on here =)
nekronavt ... well, if I could figure out how to upload a blend file with this new (to me) forum interface, I'd be happy to do so.
I think I found the problem. Noticed this dark section of grid and when I turned the model to a front view I can see that the mirror is being clipped at the edge of the darker portion of this grid. So, off I go to figure out which magical key combination I accidentally hit to cause this.
[Solved] That magical key combination was ALT+B.
