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E is for Error Message. If you've used a computer, you have come across one of these. Probably every user of Microsoft Windows has groaned as an (American) STOP sign has appeared in the middle of a box in the screen, with the words "[Program] caused a General Protection Fault in module [XXXX] at [address]", or the information that "This program has performed an illegal operation and will be terminated", which makes your software sound like a wayward surgeon facing the death sentence.

Why do we get error messages? Because to test programs completely is far too time-consuming. Once a program has more than a million lines of instructions in it (and Microsoft Word now has 2 million, though the original was just 27,000) it is impossible to test all the possible conditions under which it might operate, and the paths it might take, before the universe ends. Even Microsoft programmers would regard that as a slight delay, so programs leave the factory with error messages built in to indicate where your path has turned into a dead-end.

However, the real trouble with these messages is that their helpfulness is usually in inverse proportion to the severity of the problem. If the printer is out of paper, a smart system will tell you so. But when something goes seriously wrong, it just says "System error - restart your computer." This is usually because the fault has tripped up a more fundamental part of your machine, such as the operating system (rather than just a single program), rendering it less able to communicate.

Will error messages always be unhelpful? If - or when - computers become self-aware, they might be able to explain what's troubling them about a piece of software. But be prepared to live with error messages for a good while yet.