The Invisible Walls are bugs, that appear in several Tomb Raiders.
They stop Lara from going through what appears to be clear passage.
An invisible wall could be found in Tomb Raider III, in the Level Lud's Gate. It would stop Lara from collecting the secret in the big chamber with the second Underwater Propulsion Unit. This bug is activated because the player has saved or loaded a game in the period of time between disembarking off the first UPV and getting the second. Try not to save or load at all in this period, and the bug will not be activated.