expired ticket

The very moment before you goe in a Roman Bus, you need a bus ticket.
And I really mean it. That's because you can't rely on the onborad machine selling tickets.

From my last 2 experiences:

exp 1: on bus number 716, I get on without ticket, confident to buy it on board. I did get one. Still I inserted a 2 euro coin, just a second before reading on the box itself :"questa macchina non da resto". (trad: "this machine doesn't give back change").
Result: 2 euro paid for 1,5 value.

exp 2 : on bus 490. This time I have my 1,5 coins in my pocket.
Coin inserted, ticket received. So far, so good.
Now, I move to the opposite machine, which actually should timestamp my tickect, validating the begin of the 100 minutes time.
Ticket inserted, a strange noise heard. Ticktes gets out the machine with no timestamp.Since There's no assistance on board, and it's not allowed to talk to the driver, I just forget about this.
I left he bus to get the underground at flaminio.
When at the entrance of the underground, I'm not allowed to get in. Now I finally check what's printed on the ticket I got from the 490 bus: :"Emiss:
01/03/2012" (trad: issued on 1st may 2012). Since we are at the end of may 2013, my ticket is one uear and 2 mouths old, thus, not valid. result: no value for the 1,5 euro I paid.

With these experiences of mine I've exposed here, when in Rome, I'd suggest you to get your bus tickers in advance.