Up until recently I have had no issue with the Dock. I always leave it showing and don't have auto-hide enabled. Now whenever I switch program from Chrome to Skype or Terminal to System Preferences the Dock hides, even though I have never changed the setting to turn it on.
So far I've tried to fix this by right-clicking on the Dock separator and click Turn Hiding Off but it just changes back to Turn Hiding On once I switch programs.
I've also gone into System Preferences and selected "Automatically hide and show the Dock".
I've also killed the dock to restart it which was suggested here.
I think I've run out of options to fix this. I'm not even sure why or how it started.
Best Answer
As EDP pointed out Command+Option+D does this. The Spectacle app has shortcuts near this (+F for example) and I'll tap 'D' accidentally from time to time. Even if you're not tapping the shortcut inadvertently, Command+Option+D is a great way to switch the setting back when it's goes wrong.
Cool trick: open [Settings > Dock], and tap [Command+Option+D] a couple times.