During live there was a bug.  If you were on a zone wall and the mob was at the bottom of the wall or out a little from the wall, your bolt would shoot down, then curve down steep right at the end and do double damage. 
 
I believe P99's Devs fixed this but broke it too.  I am unsure.... 
 
Only a Dev/Admin can chime in to address this for sure. 
		
		
		
		
		
		
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