I upgraded to 3.4.1. I found that when SCI_SETCARETLINEVISIBLE is enabled, horizontal scrolling will cause random lines to be highlighted in the area that is scrolled in. It's best to see the screenshot. Only line 17 is supposed to be highlighted.
This appears to only affect OS X 10.9 because I couldn't reproduce the problem in 10.8. I also tried compiling with both Xcode 5.0.2 and Xcode 4.6.3 and their respective SDKs and the problem exists regardless of which SDK it's compiled against.