When used within a course, print_header_simple() generates build_navigation warnings if its used without navigation paramaters, as seen in require_login():
print_header() was sent a string as 3rd (CourseEval -> ) parameter. This is deprecated in favour of an array built by build_navigation(). Please upgrade your code.
line 2324 of lib/weblib.php: call to debugging()
line 2765 of lib/weblib.php: call to print_header()
line 1907 of lib/moodlelib.php: call to print_header_simple()
line 42 of user/index.php: call to require_login()