[spolsky]

In the United States two people look at your boarding pass: the TSA (at the entrance to security screening), and the airline (while boarding the plane).

The TSA also checks your ID. In most airports, the TSA is NOT online and merely looks at the printed boarding pass to make sure the name matches what is on your ID, your flight is for today, etc. In most cases the airline does not bother checking ID again, assuming that the TSA checked it.

You would not get on a plane with a fake boarding pass, but you do get into the secure area.

Because IDs are not checked by the airline, forging a boarding pass would allow you to board a plane with any name you want on the ticket -- the name on the ticket doesn't have to be your "real" name (it doesn't have to match your photo ID). This means that the TSA's various "no fly" lists, which are just lists of names of people that they've compiled that are "too dangerous to fly", are easily defeated.

[ams6110]

You do need a photo ID matching your ticket, but that is probably not a very high bar for a dedicated person to surmount.

[gergles]

Not in the US you don't. Buy a ticket under "John Doe." Retrieve boarding pass. Print a copy. While you still have it open, photoshop it (or edit the HTML, whatever) to say "Bob McTerrorist". Show your photoshopped boarding pass and your "Bob McTerrorist" license to the TSA. Clear security. Discard your photoshopped boarding pass. Retrieve the boarding pass copy from step 3 above. Present it to the gate agent and board the plane.