If you’ve gamed long enough, you’ve probably been subjected to the “bait and switch” at least once. The GM has given you the campaign pitch, you roll up characters according to that pitch, and then you find out that the actual game is very different than the one you’ve been playing. Sometimes this is obvious at the start (“this campaign is more about noble court intrigue than combat” followed by three sessions of non-stop combat) or becomes apparent as the game progresses (you create American Civil War Union soldiers for a “historically accurate campaign” only to discover six sessions in that the Confederates are being supplied new weapons by the Sontarans).
Sometimes, the bait and switch can be fun for all involved. I once ran a fantasy campaign set in Dark Ages England (there was magic, but all PCs were human). As the campaign progressed, the players discovered that they were actually playing a historical World of Darkness game, and the PCs were unknowingly bloodbound to the local bishop (and the werewolves they’d been fighting were actually trying to rid the Midlands of vampires and ghouls). The players really enjoyed this, and many were disappointed when it wrapped quickly due to outside pressures.
On the other hand, a bait and switch can be frustrating. I once pitched a campaign in which the PCs were designing military officers preparing for the USA’s first interstellar flight. They made their characters with this in mind. During the first adventure, the PCs underwent a battery of tests, including a five-year stint in suspended animation. While they were “sleeping,” Earth was engulfed in a world war and the PCs didn’t wake up for over a century. The players suddenly found themselves in a post-apocalyptic campaign (this was d20, so I ended up rewriting character sheets because of suddenly sub-optimal choices). While the players had fun, it wasn’t the campaign they signed up for and I (rightly) caught some flack for it.
So what say you? Where would you draw the line on the bait and switch? When is it acceptable and when does it cross that line?