Weird glass artifact I was messing with UV unwrapping, and this is what happened:



I reset the UV map, but to no result. For reference, I have older render of the same glass (the version which was unfortunately overwritten):



How to make it smooth again?

It looks like there are two faces in the same place oriented other ways. 
That's how face orientation view looks like:
How to rotate these faces?
Ok. The solution was to recalculate normals by using Shift + N . The problem was, that faces were facing wrong way. Blender renders fronts of faces in higher quality than backs, and mine were facing front and back in checkerboard, so that explains checkerboard thing. Recalculating normals rotated  them the right way. I think that they rotated when I was messing with UV unwrapping.