"If you meant "If the format string contains a '%'" then there is nothing wrong with it: format string contains `%` often and often."

char buff[10];
char source[]="%frog";

//This copies the string ok:
sprintf(buff,"%s",source);

//This attempts to use a non-existent floating
//point argument:
sprintf(buff,source);

In the second example the source string is treated as the format string meaning that it's contents are significant to sprintf(). So, if the source string contains a '%' character you would probably have a corrupt stack on an average PC 'C' implementation and at minimum unexpected behaviour on an 8051 'C' implementation.