User makes a 0.01 payment using a 0.02 coin in a no fee transaction. They get 0.01 change.  The TX is low priority.
(TX: c9bdcc0ba07812c76e6c6635049f0b55b628a4c3321b02ab4317d79273861232)

Two days later they make a 36BTC transaction, they include a nice fee.  The change gets used.  The TX is hung waiting on the unconfirmed 0.01 contributor. It's something like an hour old now and it's still unconfirmed as I write this. Based on the ages of the other unconfirmed TX like the blocking one, it might take seven days to clear.
(2e74980e03fc0e3ca3c73379f07661afada17cf2103e196bb59345409a6945c8)

Maybe the client could try harder to help the users avoid spending unconfirmed coin, but it seems to me that the miners should group all the TX involved in a dependency, add up the sizes and the fees, and apply their criteria to the group as if it were a single TX.

Otherwise we can expect more holdups as the QOS for no-fee transactions continues to decrease.