[Example:] A user is about to enter his username and password for a program or website when he receives an instant message. Some instant messaging clients will immediately grab the keyboard focus and move it into their own response field, because they assume the user will want to respond to the new message immediately. In reality, the user may be astonished to find that they have just typed their password into their IM client and sent it to their friends. This conflict arises because the two programs are not aware of each other’s existence, and cannot easily determine when they might get in each other’s way. To avoid such conflicts, operating systems may restrict the interaction of different programs, for example by preventing the IM client from stealing the focus.

Via the Wikipedia page. Kudos to Alex King for making me aware of this principle.