Keyloggers exist. Ergo, you shouldn't ever type an important password into a machine that doesn't belong to you. Fortunately, as others have pointed out, it is

Keyloggers exist. Ergo, you shouldn't ever type an important password into a machine that doesn't belong to you. Fortunately, as others have pointed out, it isn't 1990 anymore and I carry a mobile computer wherever I go, disguised as a phone.