Thin vertical features get wonky?

I have tried printing the reef shark
[www.thingiverse.com]
which has a thin fin extending in the z direction. I've noticed that slic3r does some strange things to the fin. There are unconnected protrusions at the thin edge of the fin as the fin narrows (increasing z) .

The pics use repetier for visualizing both the model and the stl/finished item. I've confirmed that these protrusions do exist in the printed model and that Repetier isn't changing the output or visualization.

I've noticed this on other models with thin x/y features. I've tried messing with the settings a little but nothing has changed.
Has anyone else figured out how to prevent this, short of scaling up the print?

Thx

Using Slic3r I think this is because the feature is getting smaller than your novel diameter. I am running a large 0.66mm and any thin wall tests for 0.5mm do things something like that.