When a sprite with a transparent background ends up right up against a sprite that is filled up to the shared boundary on the spritesheet there sometimes ends up being an artifact on the sprite with the transparent background. This does not show up on all browsers, but is clearly visible in these images.
It's hard to create a simple case. The problem has shown up in multiple games we are working on. Since they are a client's games I can't post a link to the games on the forum, but I can PM a link if that will help.
The workaround is to insert transparent frames on the side of the sprite that is showing the artifact, but that isn't always practical.
<img src="https://dl.dropbox.com/u/57899112/sr_artifact_bug.PNG" border="0" />
<img src="https://dl.dropbox.com/u/57899112/mp1_triangleproblem.PNG" border="0" />