I noticed that when using any brush size larger than 1x1 pixel and passing the brush over a section of line (without actually drawing) the brush image actually distorts the line you pass it over and it remains that way until you pass it over again, in which case it doesn't revert but simply is distorts the line again. The distortions are actually very miniscule and they really only flop around a few pixels here and there, but if you are doing something very detailed, you could get aggitated when your fingernail suddenly grows an extra pixel. :) this probably goes for 1x1 pixel brush size as well, but it's almost unnoticeable in that instance.

Are you using zoom snap mode? If the zoom isn't at a multiple of 100%, then that'll happen. Paintbbs/oekakibbs' solution? Don't allow those zooms at all... I personally think it's useful sometimes.

i never noticed it in oekaki and i don't really use paintbbs at all.
When i encountered the problem i wasn't using snaps and was zoomed in a little, but it countinused to happen when i turned back on the snap and set the zoom to standard... so...