UA does limit seats sometimes on SFO-SYD,LAX-SYD,ORD-HKG legs and I would imagine LAX-MEL (eff. 12/4/99) for weight/cargo reasons. I have seen ground staff bump 10-20 people just for these reasons.

Years ago, when I was based in RSA, SA
(South African Airways) had seat limits on its old JNB-TPE flight. They used to be one of the largest operators of the 747SP, the short funky looking 747. These were designed for long range operations before the days of the 747-400 and 777's. I think only 30+ were ever built?? SA phased these out and leased them to Air Mauritis and others?? They now operate 747-400's mainly. I would also think the JFK-JNB flight (the worlds longest with the LAX-MEL running a close second) would have seat/weight/cargo restrictions??