What causes gaps in small region fills and how to prevent them? I saw this cool Nuka-Cola bottle opener. I made it:

  

As you can see, nasty gaps have appeared in the text. What could've caused them? I am sure I set the filament diameter correctly. So why did the Slic3er not generate enough paths here to fill the letters properly?

I used Slic3er and Repetier Host. My printer is Prusa i3 MK2 1.75mm. The material I used was PLA.
I had a similar issue as described in this question. Curiously, yours is almost opposite as my raised lettering was OK, but the surrounding areas were poorly filled.

I came to the conclusion that Slic3r was simply not able to properly handle the geometry in my part. I tried with Cura and had great results!