At first glance, the thief knot (known as the thieves knot in some areas) is easily confused with the square knot or reef knot. This is by design. It is said that sailors would use this knot to tie up their cinched duffle bags while they were working. Any thief who untied the knot and went through the bag would then unknowingly re-tie a square knot to make it look as though the bag had never been opened. The sailor would immediately see the difference.
How is it different from a square knot?
If you look at the ASCII representation of the knot below, you will notice that the running ends, A and B, end up on opposite "sides" of the tied knot (in this illustration, the top and bottom). In a square knot they are both on the same "side" (i.e., both on the top or both on the bottom). Because of this difference, the thief knot will slip when under stong tension, and should never be used when safety is at stake.
____________ ______ ______ ____
........... \ / \ / .... \ / | B
__________ . \ / __ \ / . __ . / __|
\ . / /__\___/ . / \ / /
\ / /......... /____/ /
/ /__________/ / \
/ / \__________/ . \
| / \ . |
| \ __________ / . |
\ \________/__ \ . /
\ \......... \_____ \ /
/ \ \_______ . \ \ \
__/ . \ \__/ \ . \__/ \ \__________
A |.... / \ / \ ...... \
|____/ \______/ \______/ \____________
Thanks to tkil for his node on drawing knots with ascii art. It's not as easy as it looks.