The worst mistakes we those made once Hitler had power, started acting in a very aggressive manner and violating treaties, and the Western powers pretty much turned a blind eye to it, except to say "now, now, Mr. Hitler....We like you and all, but you need to be nice...this paper says so." So Hitler kept at it. The mistake was the decision to not act.
That's not exactly what was said, and it doesn't really refute the point I made. The Western powers were in the situation they were in primarily because they neglected their own national defense while trying to maintain large bloated empires. They were in a position where they had to buy time. The Locarno Pact was also considered an early stage of appeasement, and that was in 1926, 7 years before the rise of Hitler.