It's the way things are rendered in browser - I've been doing pixelated stuff by blowing up, in order to save memory and disk space, and occasionally things won't reach.
I'm assuming you're using a grid to line everything up?
Are the tiles on-grid, and integers? IE, if tiles are 10, grid points are multiples of ten, and x,y co-ods aren't something like 10.1256 , which can happen when nudging by cursor keys.