When one goes by land from Hong Kong to China - say the Lowu train crossing or the Shenzhen Bay bus crossing - after you go through the Chinese immigration, there's a X-ray machine that one needs to put their suitcases on. You never know if the machine is actually on or not, and there's no formal rule how large a bag needs to go in, and which smaller ones don't.

But if you have a suitcase full of iPads or iPhones, there's a decent chance you may be stopped.