I've seen this same message when working with HTML5 tags in a UIWebView. It appears to be triggered by fill() calls when filling a shape with color. I wasn't able to isolate the problem, although replacing hex colors in fillStyle() stopped the errors from appearing in my case.
context.fillStyle = '#666'; //causes errors
context.fillStyle = 'rgb(100,100,100)'; //no errors
context.fill(); //error appeared when this statement was executed
My guess on the source of this issue is an obscure bug in Safari's usage of CoreGraphics to render the related JavaScript API calls.