How to explain glitchy transitions after restart in Corona SDK program

Below is a simple Lua program of a shelf moving left and right across the screen. When the screen is tapped, the shelf is supposed to reset and start at the left again. But after that, the transitions.to do not work properly, as the shelf starts "jumping" back and forth. Can someone please help me fix this issue?

local shelf

local function movingShelf() 
    shelf = display.newRect(56.5,250,85,10)  --display shelf

    local moveLeft --move shelf
    local function moveRight()     
        transition.to(shelf, {time = 5000, x = 261, onComplete = moveLeft})
    end
    moveLeft = function()      
        transition.to(shelf, {time = 5000, x = 57.5, onComplete = moveRight})
    end
    moveRight()
end

local function restart() -- restart the program
    shelf:removeSelf()
    movingShelf()
end

movingShelf()
Runtime:addEventListener("tap", restart) -- tap to restart   

The transitions you initiate in moveLeft() and moveRight() are applied to shelf and you haven't cancelled them. This can cause this kind of glitchy behavior.

To properly remove a DisplayObject, you need to cancel any transitions it is involved in. In your restart function, you should do transition.cancel( shelf ) before removing the DisplayObject with shelf: removeSelf(). Finally, add shelf=nil for good measure (doing so is the recommended practice when removing DisplayObjects in Corona to avoid memory leaks).