Just reaching around behind me on a light aircraft to get something off the back seats can cause a wobble. Generally the smaller and lighter the aircraft the more likely weight imbalance will cause a problem. And as has been stated above by worldtrav, some aircraft like the 737 have been lengthened SO many times that there can be real challenges getting the load distribution within the CofG envelope.

When I was very young and travelled long haul overnight on the stretched IB DC-8s, there would invariably be queues in the early morning right at the back to use the loos. Many times we would have to be ‘queue managed’ back to our seats by a combination of flight crew announcements and cabin crew shooing!