I once carried a case of beer, my usual assortment of tools (15mm wrench, small pump, tire levers, tube), 2 mini u-locks, wallet/cell and stuff and a 3-ring binder in my Banjo Brothers bag. Then I rode up a really long hill with it.
That pretty much blew.
EDIT: The beer was Brooklyn Black Chocolate Stout though, so it was worth it.