Using nothing but the platform behaviour and solid sprites as a bare bones platform environment seems to result in tiny errors in the collision detection.
I'm not entirely sure how to best describe it, so here's a screenshot.
As you can see, the player sprite is inside the floor by a pixel or so, and about a pixel away from the wall, even though it should be touching the wall.
As the player moves around, it seems to be random as to when it falls into the solids like this. You can jump up and down on the spot, and each time you land it could be either sitting perfectly on top of the solid, or sitting inside it by a pixel or so.
Does this happen for anyone else?
edit: FYI, it happens in Canvas2D and in WebGL, with Point or linear scaling and in crop or scale mode.