I've never encountered a problem like this when using the app and/or passbook on the iPhone. In fact, I still have boarding passes in passbook from December that I can pull up and every flight this year since then.

I usually double check my mobile boarding pass while I'm passing the chickens just to make sure.

Another cool feature of passbook is it will send you an alert (just like a text message) with an updated boarding pass if the gate changes. Then when you pull up the BP the new gate will have a red circle around it so it stands out.

Good point about transferring BP to passbook. I thought this was automatically done and only recently discovered that you have to send each BP to passbook manually. PITA.

However I always print my BPs when I can as you never know when your phone or app will freeze up. Also I like to pack my phone away before security checkpoint - easier to lose or misplace if you have to show it to the TSA checkpoint drone as you can't carry it through the metal detector.

I also always print one too. What if my phone dies or something goes wrong with the app.

I usually take a screenshot of the mobile BP, just to be safe. That way I've also got something to show if the credit doesn't post.

The gate agent still has a printer IIRC. :p

I have become happily reliant on the mobile app and passbook, but I do see a couple of issues.

I do a lot of quick turns, where I'm checking in for my return flights before I have completed my outbound. Somehow this screws up/wipes out some of my old boarding passes, so that at the end of a trip there are only three boarding passes left in passbook when I flew four segments. This has never been a problem other than from a record keeping perspective. I get rid of the old boarding passes as soon as the flights have been credited anyhow.

The other problem was somewhat arcane, but it did require I get a paper boarding pass. It happened on the second (and last) segment of a return trip. I was checked in and had my boarding pass in passbook. The flight was late and my upgrade had not yet cleared. When I reached the gate I found out that my upgrade did clear, but I was unable to "reprint" the boarding pass to passbook. Itineraries seem to drop off the reservation list on the app home screen as soon as the scheduled departure time of the last flight passes. Because this had happened before my upgrade cleared, I couldn't figure out how to get to "reprint boarding pass" tab I usually use to update a boarding pass. I ended up just getting paper from the GA, so not a serious problem, but it shouldn't have been necessary.

IME, having an Amtrak segment in a PNR has kept mobile BP's from showing up.

i do like having a mobile BP, but i still always, always make sure to take two minutes and print out a physical BP from the kiosk. in case of irrops, flight changes, inadvertent seat mixups, missing flight credit and a whole slew of other issues, i've found that nothing really beats having a spare physical copy in your backpack as an insurance policy.

If everyone prints out a bp anyways, why bother with the mobile boarding pass in the first place?

I don't print mine out, it defeats the purpose of even going though the trouble of using the app.

I have used the mobile BP perhaps 100 (or more) times since iOS6 came in September (plus more before passbook) and only had a handful of issues (all caused by my battery running out). I've never not had miles post and never had issues with the readers (I am sure others have).

I think taking a screen capture or printing a paper copy is just belts and suspenders...and who wears suspenders these days.

As do I "just in case"

But of there's a problem at the checkpoint where your mobile bp won't scan, you will be sent back to get a printed one either hatched from a chicken or printed by a lobby agent which is why I agree with enricong above