$ ./ffscreencast
Available devices:
[1] Capture screen 0
[2] Capture screen 1
[3] Capture screen 2
Enter device number: 2
ffmpeg -hide_banner -loglevel info -thread_queue_size 512 -f avfoundation -video_size -i "2" -c:v libx264 -crf 0 -preset ultrafast -threads 0 "/Users/bparks/Desktop/Screencast 2015-10-26 at 12.33.37.mkv"
Option thread_queue_size (set the maximum number of queued packets from the demuxer) cannot be applied to output file 2 -- you are trying to apply an input option to an output file or vice versa. Move this option before the file it belongs to.
Error parsing options for output file 2.
Error opening output files: Invalid argument

Using FFMpeg 2.7.2 on OSX Yosemite (10.10.5)