Line being displayed under fixed size canvas

May not be consistently reproducible but photoshop creates artifacts at the bottom and right side of the canvas after scaling an image inside the canvas, giving the impression that other objects on the canvas are extending outside/inside its boundary.

Steps where this occurred: 

Create a new document at 240 x 240 pixels.
Create a square block 24x24px to use as a means of setting guides on each side of the canvas.
Drag an image of larger scale onto the canvas and then scale it down to fit.
Zoom in, then zoom out to 100%.