Jetstar charged me $160 extra to check in a small light weight nylon string Guitar, but then on arrival in NZ - Christchurch... low and behold, no Guitar! Well the Jetstar baggage counter staff in Christchurch were very young and seemed more confused than I was with what to do about it. Luckily with help from an older security guard I filled out a form with my contact details and hoped food the best. I then spent 2 days trying to track it down with and then had to wait at one of our holiday destinations longer than we wanted for the Guitar to be couriered to me.
I think the reason the Sydney Jetstar staff didn't get the Guitar on-board is because the same staff members that checked me in at the terminal counters, then checked me in at the gate (which was a long walk away) and then the same staff were the air hostesses on the flight.
So the classic budget airline case of..... too little staff doing too many duties!