%% appears to steal an argument from the s/printf()
argument list.  When %% is the last %-construct in the
format string, the lack of an argument is silently ignored.
In other cases, unpleasant things happen:

printf("%d %%",1);
1 %

printf("%d %% %d",1,2);
Warning:  sprintf: too few arguments

printf("%d %% %d",1,2,3);
1 % 3