Ever double kiss a cue ball? Would you even know it if you did? Below are links to various clips around double kissing and a high speed video (play in slow motion from the options for the ultimate view) of what a double kiss ACTUALLY is. Also, double kissing the cue ball is a ball-in-hand foul in 8 Ball Express.
Speaking from experience, if you suspect a double hit is going to occur, call a neutral party to watch the hit BEFORE it happens to avoid an argument after the hit.
CLICK FOR VIDEO EXAMPLES:
Multiple Clips/Links: (Thanks Dr. Dave!) http://billiards.colostate.edu/threads/fouls.html#double
High Speed Video Showing Mechanics of Double Hit: http://billiards.colostate.edu/threads/fouls.html#double
Double Hits, Frozen Balls and Push Shots:
Double Hit Example (Not the best, but I just like to laugh at how he says"foul"):