The phrase is derived, not from the position of premium bottles in liquor stores, but their position behind bars. (as opposed to "well" liquors, which are stored in the bartender's speedrack well, and "call" liquors, which are popular but not displayed (and which must be "called").
The best of the best. Usually refers to where wine or liquor are kept in a specialty liquor store. The best stuff is kept at the top for display until someone needs it.
When one takes a crap in the water tank of a toilet. This is usually done as a party prank or as a method of smiting one's enemies. It's a real pain to clean out, stinks like hell, and makes the toilet flush brown for days.