That's why you see this custom error page ;-)
File "/Users/irmen/Projects/www_snakelets/snakeserver/YpageEngine.py", line 263, in createOutput
tplpage.create(out, _request, _response, templatedPage=self)
File "<Ypage_code_of_Template>", line 23, in create
File "<Ypage_code_of_templatepage_error>", line 14, in create
ZeroDivisionError: integer division or modulo by zero