Tk produces "bad screen distance" in Tk_GetScreenMM (convert string to screen millimeters) and TkGetDoublePixels (convert string to number of pixels) when strtod fails on the string being passed. It also produces the error in SetPixelAny (convert object to pixel) if the string doesn't start with a double, and SetMMFromAny (convert object to millimeters) if the string either is no double, or not followed by "[cimp]". Finally, TkPixelParseProc returns the error if TkGetDoublePixels returns a negative number.

In relationship to the canvas line command, TkPixelParseProc is used for converting the -width, -activewidth, and -disabledwidth arguments. None of these are used in the test case, though, so it's not clear which pixel parsing fails specifically.