thank you all. just wanted to be sure before i say something to someone at Changi-- which i will do the next time i'm there. i've never actually heard of problems with this policy before- i don't find the SKL matrons the friendliest in the world but they're usually on top of the rules.
anyway, this was a funny trip for my sister.
she flew HKG-SIN and then was turning around and going SIN-HKG-ORD the next morning (we buy B-class fares out of SIN as they're quite a good deal...). anyway, you can have UA pick up and retag the bags while you hang out at the lounge overnight (in her case the premier lounge!!) but this time they forgot to remove the SIN tag from the night before so her bags were loaded on in SIN in the morning but then turned around when they got to HKG and ended up back in SIN that night. needless to say the 1k baggage line was VERY confused about what happened and it took a good 48 hours to sort it out.