I've done a lot of bouldering, sport climbing and some aiding. Awhile back I went ice climbing with some guys for about a week. I didn't participate much simply because I didn't feel the confidence I normally do in the equipment. Particularly ice screws.
I asked the question, "Under what circumstances would you abandon a screw you just placed?" I did not get total consensus on the circumstances that would need to be present in order for a screw to be abandoned for use. The discussion became so heated I abandoned it completely.
Answered I received:
When there is a long vertical crack that is formed when placing it.
When a short horizontal crack is formed during placement.
When a star crack created when placing it.
I will say that a star fracture did receive some consensus among the crew. A few of these guys have been climbing for a long time and their practices seemed safe but none of them had any physics degrees either, not that I do.
My question, as above is, under what circumstances would you abandon a recently placed ice screw?
As well, how far away from the crack would you have to go to place the next one safely?