Here are the so-called fence bugs ("fence faults"), which are nothing, but faulty fittings, invisible joint gaps through which you can move (jump) back and forth. For instance, when compelting "Return to Egypt" level of TR1Gold, this could be a way of taking hands on the Uzis. And a typical example from TR2, Monastery of Barkhang, where you can flood the room with water.

You can have a glance on one or two animated gifs: