Your best bet is to probably use the white list for your do not pass keys instead of a black list. This way you tell very specifically what keys you want to go through.

Most likely what is happening, is you are pressing shift, or ctrl or alt + a movement key. This will cause the behavior you specify (at least, I know it did for me). I switched to using white list and haven't had the issue since.

[Edit]Forgot to add the alternative if you're not down with white list. If you stick with black list, you'll have to add every combination of the aforementioned modifier keys (shift + left, shift + right, ctrl + left, etc). Imo this ends up bloating the list a bit much, and at some point you end up having more keys black listed then not.[/Edit]