Turns out that some people playing through the Wii version find themselves in Thailand without a lever, a lever that, I assume, opens something that lets you continue through the game.
Eidos confirms that the bug exists and sends their apologies:
"We are aware of a bug which is present in the Nintendo Wii version of Tomb Raider: Underworld. We would like to stress that this is an extremely infrequent bug, however on the rare occasion that it occurs; it affects the presence of a lever in the Thailand level of the game and prevents progress beyond this level. Eidos sincerely apologies to anyone who has experienced this frustrating problem.
The quality of our games is paramount to us and the Nintendo Wii version of Tomb Raider: Underworld went through three separate QA testing departments prior to release. Regrettably anomalies such as this do occur occasionally in videogames despite the best efforts by publishers to avoid them. The good news is that it is not an unsolvable bug, so if you encounter it and it persists please visit www.eidosinteractive.co.uk/support/worldmap.html ."
What happens when you go to that website listed? You see a sea of support numbers to call. When you do I'd assume they tell you to revert to an earlier save or hit you up with one of their saves right past that point. Hmmmm, does anyone else smell a recall?