Apparently OS X is very friendly to folks with vision and hearing problems. However some of these features can be engaged by key-combinations. Inadvertently
This handy one, that sets the individual sound output of headphones or built-in-speakers on a MacBook Pro to Mono vs. Stereo, messed with me, for 3 days of rechecking my entire Mac Pro setup - as I could not figure out why the Mix sounded “stereo” on the built-in speakers, but phase canceled or etc… in headphones. While panning left-right did work.

https://discussions.apple.com/message/21233898#21233898

Good way to check if your song will sound okay in mono, but if engaged, can cause mild confusion. As well as, the input in question, needs to be plugged in - so if headphones are disconnected the tick-box is clear until the audio is re-routed to the headphone jack etc..