I've encountered a bug of some kind. Not sure if it's just an issue with the demo, and I'm not sure if it's the same on every canvas size, but I'm using a 16x16 canvas.
On certain zoom levels there are pixels that become impossible to colour in. The pencil tool acts as if they're not there (the colour preview isn't even visible when hovering over them).
If I slide the zoom level in and out I can sometimes hit a point where the pencil now recognises one of the "dead" pixels and I can colour it in.. So I'm having to constantly zoom in and out trying to find zoom levels to fill in the dead pixels.