What are some of the practical examples that require back-tracking (for whatever definition of "require")? The example I was once being told about was parsing assignment expressions:

    Expr ::= CanBeLhsOfAssignExpr | CanBeLhsOfAssignExpr "=" Expr | CanNotBeLhsOfAssignExpr

but in practice, it's easily parsed in the following manner:

    def parse_Expr(self):
        # parse_ExprInner parses CanBeLhsOfAssignExpr or CanNotBeLhsOfAssignExpr cases
        result = self.parse_ExprInner()

        if self.peekToken() == TOKEN_ASSIGN:
            if result.is_CanBeLhsOfAssignExpr:
                token = self.skipToken()

                # assignment is right-associative
                rhs = self.parse_Expr()
                result = self.make_AssignExpr(token, result, rhs)
            else:
                self.raiseParseError("{} can't be target of assignment", result)

        return result

The idea is to parse the prefix under "relaxed" rules, delaying the well-formedness check until it's actually needed.