The t+ carnets which can be purchased ahead of time to give convenience and some discounts for Paris transport have a high rate of demagnetization. This means ticket discounts are wiped out and embarrassing situations results. For the Metro, you can go to an attendant. For a bus, where people are pushing on behind you, the situation is much difficult.
There seems to be a move to use a card that you can buy and load / top off but this reduces the discounts.