I just had almost the same thing happen...sometimes there just isn't an explanation. First I would check to see that you don't actually have two copies of the letters on top of each other or one of them doesn't have an open shape. I think sometimes if you perform functions in other programs it brings these inexplicable anomalies into ArtPath. For example if you merge, combine, expand, etc. in those other programs it may create these issues. Another thing could be loops in the paths that you really can't see. Also, sometimes PDF conversions will do the same thing but you'll never be able to identify it with certainty. You just have to inspect your file very closely each time before actually finding out when it's run and ruining the material....just like I did this morning!