It comes up as a pop up, but I only saw CRC Error when it turned blockland black and hid the pop up behind blockland. I couldn't read it well because every time I tried to move the blockland window, it did nothing.
I tried clicking the pop up at the tab bar, it showed only Fatal Error..., but it would come up one millisecond and then disappear behind blockland. The pop up just died when I closed blockland.