One bug I experienced is that, in fullscreen, the zoom buttons in the lower right corner don't align visually to their apparent boxes. It seems like the zoom in button box occupies the right half of the zoom in button visual, and the zoom out button box occupies the left half of the zoom IN button visual. It seems like the scaling for the "mouse inside rect" does not match the scaling for the visual buttons.
Another bug I encountered was backspacing to undo did not fully undo a paint operation. It seemed that portions of a paint operation was immune to undo if it was inside a stamp.
One last bug, I'm not sure if it is, was that on Windows 7 (feel free to ignore this one lol), the "fullscreen" was actually just fullscreen windowed, not fullscreen proper.