Click between code and console window. Code window stops responding and the below error appears in the console. Requires shutting the code window and reloading the file.
2016-06-04 19:51:04.413 R[25124:2372223] !!! _NSLayoutTreeLineFragmentRectForGlyphAtIndex invalid glyph index 866
2016-06-04 19:51:04.414 R[25124:2372223] *** RController: caught ObjC exception while processing system events. Update to the latest GUI version and consider reporting this properly (see FAQ) if it persists and is not known.
*** reason: *** NSRunStorage, _NSBlockNumberForIndex(): index (866) beyond array bounds (866)
*** name: NSRangeException, info: (null)
*** Version: R 3.3.0 (70573) R.app R 3.3.0 GUI 1.68 Mavericks build
Consider saving your work soon in case this develops into a problem.