A YACC parser can work even if it has shift/reduce or reduce/reduce conflicts. Please refer to the YACC documentation available from our web site (Section 5 page 11). Some of these conflicts may lead to an unexpected behavior of your parser whereas others are safe.
The original grammar has been revised in order to limit the number of shift/reduce and reduce/reduce conflicts with YACC. However, some of them remain but are considered are safe. Here's a description of these changes w.r.t. the original grammar.