The console window is visible at the top of the screen, with the code ready to be entered.
unstick_player is a console command in Thief Gold version 1.9. The console is accessible by pressing Ctrl+P. When entered, this command forces Garrett to perform a climbing action, as though he were climbing atop an obstacle. As the name implies, unstick_player is used to "[a]ttempt to free player when stuck."
This command can also be used to access areas of the 3D environment that are normally impossible to reach without the use of a map editor. When performed in rapid succession (i.e., about one second between each command enter), Garrett can effectively 'climb' upward through the air to reach high places. Rate of climbing is even faster when this is done in front of a wall. Entering this command while falling from a high place can save Garrett from taking damage or dying, if done before impact.
Examples of out-of-bounds locations accessed via unstick_player are shown here:
