In the "C Programming Language" (1988) by Kernighan and Ritchie, on p. 234, concerning the grammar of the C language, it reads
...this grammar is acceptable to the YACC parser-generator. It has only one conflict, generated by the if-else ambiguity.
What are they referring to? What is the "if-else ambiguity" and why does it generator a conflict for YACC?