Regarding lex/yacc/flex/bison, I remember (ca. 1980) when DEC's
compiler group first got their hands on lex and yacc. For yucks they
put the BLISS grammar through yacc. It came back with an error
message that the grammar was ambiguous. And it turned out that, yes,
Wulf's grammar for BLISS had an obscure corner case that *was*
ambiguous. That caused quite a stir.
-Paul W.