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.
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.
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.