Modes are problematic because they can result in an
error when the user not aware of what mode the interface is in, performs an action that is appropriate in a different mode, and gets an undesired response. A mode error can be startling, disorienting and annoying as the user copes with the violation of their expectations. Problems occur if a change in the system state happens unnoticed (initiated accidentally, by the system, or by another person), or if after some time the user forgets the current mode. Another common problem is a sudden change of state that interrupts a user's activity, such as
focus stealing. In such a situation it the may perform operations with the old state in mind, while the brain has not yet fully processed the signals indicating the state change.
Common examples ; Keyboard
lock keys: Such as
caps lock,
num lock,
scroll lock, and the
insert key. ;
Dead keys for
diacritics: Create a short-term mode, at least if they do not provide visual feedback that the next typed character will be modified. ; Multiple keyboard layouts: Users whose language is not based on the
Latin alphabet commonly have to interact using two different
keyboard layouts: a local one and
QWERTY. This gives rise to mode errors linked to the current keyboard layout: quite often, the synchronization of "current layout" mode between the human and the interface is lost, and text is typed in a layout which is not the intended one, producing meaningless text and confusion. Keyboard keys in user interface elements like "(y/n)" can have opposite effect if a program is translated. ; Modal dialog while typing: The sudden appearance of a
modal error dialog while typing, which is a form of
focus stealing. The user expects the typed text to be inserted into a text field, but the unexpected dialog may discard all the input, or may interpret some keystrokes (like "Y" for "yes" and "N" for "no") in a way that the user did not intend, often triggering a destructive action that cannot be
reverted. Programmers can mitigate this by implementing a short delay between the modal dialog displaying and it beginning to accept keyboard input. ;
vi text editor: Is challenging for many beginners because it uses modes. ; Control vs. messaging: In multiple video games, the keyboard is used both for controlling the game and typing messages. Users may forget they are in "typing mode" as they attempt to react to something sudden in the game and find the controls unresponsive (and instead their text bar full of the command keys pressed).
In transportation accidents ; Air France Flight 447 crash: Mode confusion was part of the events that led to the loss of
Air France Flight 447 in 2009, and the loss of life of 228 people. The pilots reacted to a loss of altitude by pulling on the stick, which would have been an appropriate reaction with the autopilot fully enabled, which would then have put the aircraft in a climbing configuration. However, the airplane's systems had entered a mode of lesser automation ("direct law" in Airbus terms) due to a blocked airspeed sensor, allowing the pilots to put the plane in a nose-high stall configuration, from which they did not recover. ; Asiana Airlines Flight 214 crash: According to the
NTSB, one of the factors contributing to the 2013
Asiana Airlines Flight 214 crash was
"the complexities of the autothrottle and autopilot flight director systems … which increased the likelihood of mode error". ; Red7 Alliance collision: On January 17, 2015, the offshore supply vessel "Red7 Alliance" collided with a lock gate of the Kiel Canal in Germany, damaging it severely. An investigation concluded that the levers controlling the ship's
Azimuth thrusters were not used in a way appropriate to the mode they were set to, resulting in the ship accelerating instead of coming to a stop in the lock. ; USS John S. McCain collision: On August 21, 2017, the US Navy destroyer
USS John S. McCain collided with a commercial tanker in the Strait of Malacca, resulting in the loss of life of ten crew members. An investigation conducted by the US military concluded that immediately prior to the collision, helm and propulsion controls had been redistributed between bridge stations, and the bridge crew was not fully aware of that redistribution. ; VOS Stone collision: On April 10, 2018, the 5000 ton supply vessel
VOS Stone unberthed from a wind platform under construction in the Baltic Sea. The vessel's master decided to put the steering in an alternative mode to perform a test of the system. Insufficient communication with the officer of the watch led to a temporary loss of control, collision with the platform, injury to three crew members, and significant damage. ; F-35 destruction: On April 19, 2020, an
F-35A fighter jet was destroyed in a landing mishap at
Eglin Air Force Base. Investigations concluded that the aircraft was misconfigured with the wrong mode of
autothrottle, resulting in the aircraft becoming uncontrollable upon touchdown. ; Manawanui grounded: On October 5, 2024, the
New Zealand Navy hydrographic vessel Manawanui ran aground on a reef off
Siumu,
Upolu,
Samoa, and sank the following day. According to an official inquiry, the bridge crew failed to recognize that the ship's autopilot was enabled. == Assessment ==