In Chip's Challenge 1, a thief, often referred to as a spy, removes all boots that Chip has when he walks on him, and acts as a wall towards monsters and blocks. In Chip's Challenge 2, the red thief removes all boots that Chip or Melinda have when they walk on him, and the blue thief removes all keys that Chip or Melinda have when they walk on him. Both these thieves halve Chip or Melinda's bonus points upon being stepped on. Their effects can be counteracted with a thief bribe.
Sometimes, it can be to Chip's advantage to lose a superfluous boot; see Elementary.
Microsoft Visual C++ v1.52 contains tools named Spy and DDE Spy. The icon of Spy is identical to the tile used in MSCC; DDE Spy uses the same icon, modified with a lightning bolt.
Spy About dialog
DDE Spy icon
In later versions of Visual C++ / Visual Studio, Spy is replaced by Spy++, with a modified icon:
Spy++ About dialog