The best advice is to exercise judgment when 'booking' open sectors on any carrier.

It's unfortunate that this has happended to you on QF and will influence your opinion towards QF, but this really could have happened on a whole number of carriers.

You should only avoid an airline (or any business) when they genuinely screw up. QF haven't screwed up. You had the option to make a reservation on QF when you bought your ticket (an open sector is not a reservation), or you could have at least seen (or been advised by your TA) that there is no availability between SYD-HNL and should look at a different route.