Word processors never display a cursor.
You never have to use the spacebar when typing long sentences.
All monitors display 2-inch high letters.
High-tech computers, such as those used by NASA, the CIA, or some such governmental institution, have easy-to-understand graphical interfaces.
Those that don't will have incredibly powerful text-based command shells that can correctly understand and execute commands typed in plain English.
Corollary: You can gain access to any information you want by simply typing "ACCESS ALL OF THE SECRET FILES" on any keyboard.
Likewise, you can infect a computer with a destructive virus by simply typing "UPLOAD VIRUS."
Viruses cause temperatures in computers, just like they do in humans. After a while, smoke billows out of disk drives and monitors.
All computers are connected. You can access the information on the villain's desktop computer, even if it's turned off.
Powerful computers beep whenever you press a key or whenever the screen changes. Some computers also slow down the output on the screen so that it doesn't go faster than you can read. The really advanced ones also emulate the sound of a dot-matrix printer as the characters come across the screen.
All computer panels have thousands of volts and flash pots just underneath the surface. Malfunctions are indicated by a bright flash, a puff of smoke, a shower of sparks, and an explosion that forces you backward. See #7, above.
People typing away on a computer will turn it off without saving the data.
A hacker can get into the most sensitive computer in the world before intermission and guess the secret password in two tries.
Any PERMISSION DENIED has an OVERRIDE function.
Complex calculations and loading of huge amounts of data will be accomplished in under three seconds. In the movies, modems transmit data at two gigabytes per second.
When the power plant/missile site/whatever overheats, all the control panels will explode, as will the entire building.
If you display a file on the screen and someone deletes the file, it also disappears from the screen. There are no ways to copy a backup file -- and there are no undelete utilities.
If a disk has encrypted files, you are automatically asked for a password when you try to access it.
No matter what kind of computer disk it is, it'll be readable by any system you put it into. All application software is usable by all computer platforms.
The more high-tech the equipment, the more buttons it has. However, everyone must have been highly trained, because the buttons aren't labelled.
Most computers, no matter how small, have reality-defying three-dimensional, real-time, photo-realistic animated graphics capability.
Laptops, for some strange reason, always seem to have amazing real-time video phone capabilities and the performance of a CRAY-MP.
Whenever a character looks at a VDU, the image is so bright that it projects itself onto his/her face.
Computers never crash during key, high-intensity activities. Humans operating computers never make mistakes under stress.
Programs are fiendishly perfect and never have bugs that slow down users.